JAMB UTME - Chemistry - 2025

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The molecular mass (molar mass) of a substance is defined as the mass of one mole of that substance. The relationship is:

\[\text{Molar mass} = \frac{\text{Mass}}{\text{Number of moles}}\]

Given that 0.5 mole of the alkanoic acid weighs 44 g:

\[\text{Molar mass} = \frac{44\,\text{g}}{0.5\,\text{mol}} = 88\,\text{g/mol}\]

The molecular mass of the alkanoic acid is therefore 88. This corresponds to butanoic acid (CH₃CH₂CH₂COOH), which has the molecular formula C₄H₈O₂: (4 x 12) + (8 x 1) + (2 x 16) = 48 + 8 + 32 = 88.

A common error is to multiply mass by moles instead of dividing. Remember: if a fraction of a mole has a certain mass, the full mole must weigh proportionally more.

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To determine the group and period of an element from its electron configuration, two pieces of information are needed:

• The period is the highest principal quantum number (the largest shell number) occupied by electrons.
• The group (for s-block and p-block elements) is determined by the number of electrons in the outermost shell.

The electron configuration given is \(1s^2\,2s^2\,2p^6\,3s^2\). The total number of electrons is \(2 + 2 + 6 + 2 = 12\), which identifies the element as magnesium (Mg).

The highest principal quantum number is 3 (from the \(3s^2\) subshell), so the element is in Period 3.

The outermost shell (\(n = 3\)) contains only 2 electrons (both in the \(3s\) subshell). Since these are s-block electrons, the element is in Group 2.

Therefore, the element belongs to Group 2 and Period 3.

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When air is passed through a series of reagents, each one absorbs a specific component:

1. Alkaline pyrogalol (alkaline solution of pyrogallol) absorbs oxygen (O₂). Pyrogallol in alkaline solution reacts readily with oxygen and turns dark brown.
2. Potash (concentrated potassium hydroxide solution, KOH) absorbs carbon dioxide (CO₂). The CO₂ reacts with KOH to form potassium carbonate: CO₂ + 2KOH → K₂CO₃ + H₂O.
3. Fused calcium chloride (CaCl₂) in a U-tube absorbs water vapour (H₂O). Anhydrous CaCl₂ is a powerful desiccant (drying agent).

The main components of air are nitrogen (~78%), oxygen (~21%), argon and other noble gases (~0.9%), carbon dioxide (~0.04%), and water vapour (variable). After removing oxygen, carbon dioxide, and water vapour, the components that remain unabsorbed are noble gases and nitrogen. These are chemically inert (noble gases) or unreactive with the reagents used (nitrogen), so none of the three reagents can remove them.

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Litmus is an acid-base indicator. Red litmus paper turns blue only in the presence of a base (alkaline solution), which has a pH greater than 7.

Examining the given pH values:

• pH 2.5 - strongly acidic (no colour change; red litmus stays red)
• pH 3.5 - acidic (no colour change)
• pH 7.0 - neutral (no colour change; litmus remains the same colour)
• pH 8.0 - basic/alkaline (red litmus turns blue)

Only the solution with pH 8.0 is alkaline, so it is the only one that will turn red moist litmus paper blue. Acidic and neutral solutions cannot cause this change.

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Reforming is a petroleum refinery process that rearranges the molecular structure of hydrocarbons to produce higher-octane fuels and aromatic compounds. The most common type is catalytic reforming, which converts naphthenes (cycloalkanes) and straight-chain alkanes into aromatic hydrocarbons such as benzene, toluene, and xylene, typically using a platinum-based catalyst at high temperature.

The conversion of cyclohexane to benzene is a classic example of reforming. In this reaction, cyclohexane (C₆H₁₂) undergoes catalytic dehydrogenation, losing three molecules of hydrogen to form benzene (C₆H₆):

\[ \text{C}_6\text{H}_{12} \xrightarrow{\text{Pt catalyst, heat}} \text{C}_6\text{H}_6 + 3\text{H}_2 \]

This aromatization reaction increases the octane rating of the fuel fraction and produces valuable aromatic feedstocks for the chemical industry.

The other options describe different processes:

• Converting hexene to hexane is hydrogenation (addition of hydrogen across a double bond).
• Converting hexane to carbon(IV) oxide and water vapour is combustion (burning in oxygen).
• Converting hexane to butane and ethane is cracking (breaking larger molecules into smaller ones).

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Permalloy is an alloy composed of iron (Fe) and nickel (Ni), typically in a ratio of approximately 20% iron and 80% nickel, though the exact composition can vary.

Permalloy is valued for its exceptionally high magnetic permeability, meaning it is very easily magnetised even by weak magnetic fields. This property makes it useful in:

• Transformer cores
• Magnetic shielding
• Recording heads in data storage devices
• Sensitive magnetic instruments

The other metals listed form different alloys with iron:

• Iron + tin = tin plate (tin-coated iron, used for food cans)
• Iron + chromium (plus nickel and carbon) = stainless steel
• Iron + copper is not a standard named alloy

The defining feature of permalloy is that it is a nickel-iron alloy.

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Iron is extracted from its ore in a blast furnace. The iron that comes directly out of the blast furnace is called pig iron. It contains about 3-4% carbon along with smaller amounts of impurities such as silicon, manganese, phosphorus, and sulphur.

Pig iron is brittle due to its high carbon content and is not suitable for most engineering applications in its raw form. It must be further processed to produce more useful forms of iron and steel:

• Cast iron is produced by remelting pig iron and allowing it to cool in moulds. It still has a relatively high carbon content (2-4%) but has improved properties for casting.
• Wrought iron is nearly pure iron (less than 0.1% carbon) produced by removing most of the carbon and impurities from pig iron. It is soft, malleable, and resistant to corrosion.
• Carbon steel is produced by carefully controlling the carbon content of iron (0.1-1.5%) to achieve specific mechanical properties.

The iron that comes directly from the blast furnace, before any further refining, is pig iron.

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Sulphuric acid (H₂SO₄) is a diprotic acid, meaning each molecule donates two hydrogen ions when it dissociates completely in water:

\[\text{H}_2\text{SO}_4(aq) \rightarrow 2\text{H}^+(aq) + \text{SO}_4^{2-}(aq)\]

Step 1: Find [H⁺]

Since each mole of H₂SO₄ produces 2 moles of H⁺:

\[[\text{H}^+] = 2 \times 0.06 = 0.12 \text{ mol dm}^{-3} = 1.2 \times 10^{-1} \text{ mol dm}^{-3}\]

Step 2: Find [OH^-]

Using the ionic product of water at 25 °C:

\[K_w = [\text{H}^+][\text{OH}^-] = 1.0 \times 10^{-14}\]

\[[\text{OH}^-] = \frac{K_w}{[\text{H}^+]} = \frac{1.0 \times 10^{-14}}{1.2 \times 10^{-1}}\]

\[[\text{OH}^-] = \frac{1.0}{1.2} \times 10^{-14+1} = 0.833 \times 10^{-13} = 8.3 \times 10^{-14} \text{ mol dm}^{-3}\]

Therefore [H⁺] = \(1.2 \times 10^{-1}\) mol dm^-3 and [OH^-] = \(8.3 \times 10^{-14}\) mol dm^-3.

A common mistake is forgetting that sulphuric acid is diprotic and using [H⁺] = 0.06 instead of 0.12, which would give \(1.2 \times 10^{-2}\) instead of \(1.2 \times 10^{-1}\) and an incorrect OH^- concentration of \(8.3 \times 10^{-13}\).

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For a reaction that follows first-order kinetics, the half-life is related to the rate constant by the formula:

\[t_{1/2} = \frac{\ln 2}{k} = \frac{0.693}{k}\]

Given \(k_1 = 3.85 \times 10^{-3}\, \text{s}^{-1}\):

\[t_{1/2} = \frac{0.693}{3.85 \times 10^{-3}}\]

\[t_{1/2} = \frac{0.693}{0.00385}\]

\[t_{1/2} = 180\, \text{s}\]

The half-life of radioactive phosphorus is 180 s.

An important feature of first-order kinetics is that the half-life is independent of the initial concentration - it depends only on the rate constant. This is why radioactive decay (which always follows first-order kinetics) has a constant half-life regardless of how much of the substance remains.

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This is a Faraday's law of electrolysis problem. The relationship between mass deposited, current, and time is:

\[m = \frac{M \times I \times t}{n \times F}\]

where \(m\) = mass deposited (g), \(M\) = molar mass, \(I\) = current (A), \(t\) = time (s), \(n\) = number of electrons transferred per ion, and \(F\) = Faraday constant (96500 C/mol).

For aluminium: \(\text{Al}^{3+} + 3e^- \rightarrow \text{Al}\), so \(n = 3\), \(M = 27\), \(m = 9\) g, \(I = 18\) A.

Rearranging for time:

\[t = \frac{m \times n \times F}{M \times I}\]

\[t = \frac{9 \times 3 \times 96500}{27 \times 18}\]

\[t = \frac{2\,605\,500}{486}\]

\[t = 5360.49 \text{ seconds}\]

Converting to minutes:

\[t = \frac{5360.49}{60} = 89.34 \text{ minutes}\]

The time required is 89.34 minutes.

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The boiling point of a substance depends on the strength of its intermolecular forces and, to a lesser extent, its molecular mass. The key intermolecular forces in order of strength are: hydrogen bonding > dipole-dipole > van der Waals (London dispersion).

Consider the four compounds:

• CH₃CH₂OH (ethanol, M_r = 46): contains an -OH group, so it forms hydrogen bonds. Boiling point approximately 78 °C.
• CH₃CH₂CH₂Cl (1-chloropropane, M_r = 78.5): polar C-Cl bond gives dipole-dipole interactions but no hydrogen bonding. Boiling point approximately 47 °C.
• CH₃CH₂CH₂CH₂CH₂CH₃ (hexane, M_r = 86): non-polar, only van der Waals forces. Boiling point approximately 69 °C.
• CH₃CH₂CH₂OH (propan-1-ol, M_r = 60): contains an -OH group, so it forms hydrogen bonds, and has a higher molecular mass than ethanol. Boiling point approximately 97 °C.

Propan-1-ol (CH₃CH₂CH₂OH) has the highest boiling point. It combines hydrogen bonding (the strongest intermolecular force among these molecules) with a greater molecular mass than ethanol, giving it stronger overall intermolecular attractions.

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The fountain experiment demonstrates the very high solubility of certain gases in water. A round-bottom flask is filled with the gas and inverted over a trough of water (often containing an indicator). When a small amount of water enters the flask and dissolves the gas, the pressure inside drops dramatically. Atmospheric pressure then forces water up into the flask in a spectacular fountain.

For this experiment to work, the gas must be extremely soluble in water so that it dissolves almost instantly on contact, creating a near-vacuum inside the flask.

Hydrogen chloride (HCl) is the classic gas used. It is one of the most soluble gases in water: about 450 volumes of HCl dissolve in one volume of water at room temperature, forming hydrochloric acid. Ammonia (NH₃) is also commonly used for the same experiment, but it is not among the given options.

Hydrogen sulphide (H₂S) is only moderately soluble and is extremely toxic, making it unsuitable. Dinitrogen(I) oxide (N₂O, nitrous oxide) and nitrogen(II) oxide (NO, nitric oxide) are both poorly soluble in water and would not produce the dramatic pressure drop needed for the fountain effect.

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Chlorine has an atomic number of 17, meaning a neutral chlorine atom has 17 electrons. These electrons fill the available subshells in order of increasing energy:

1. \(1s\) holds 2 electrons (total: 2)
2. \(2s\) holds 2 electrons (total: 4)
3. \(2p\) holds 6 electrons (total: 10)
4. \(3s\) holds 2 electrons (total: 12)
5. \(3p\) holds the remaining 5 electrons (total: 17)

The electron configuration is therefore: \(1s^2\,2s^2\,2p^6\,3s^2\,3p^5\)

Examining the other options:

• \(1s^2\,2s^2\,2p^6\,3s^2\,3p^4\) gives only 16 electrons (this is sulphur).
• \(1s^2\,2s^2\,2p^6\,3s^2\,3p^6\) gives 18 electrons (this is argon).
• \(1s^2\,2s^2\,2p^5\,3s^2\,3p^6\) gives 17 electrons but violates the aufbau principle - the \(2p\) subshell must fill completely (6 electrons) before the \(3s\) subshell begins.

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The empirical formula is the simplest whole-number ratio of atoms in a compound. The empirical formula mass (sometimes called empirical mass) is the molar mass corresponding to that simplest formula.

The molecular formula given is C₆H₁₂O₆. To find the empirical formula, divide all subscripts by their greatest common factor:

\[\text{GCF of } 6, 12, 6 = 6\]

\[\text{Empirical formula} = \text{C}_1\text{H}_2\text{O}_1 = \text{CH}_2\text{O}\]

Now calculate the empirical formula mass using the given atomic masses (H = 1, C = 12, O = 16):

\[\text{Empirical mass} = 12 + 2(1) + 16 = 30\]

As a check, the molecular mass of C₆H₁₂O₆ is 6(12) + 12(1) + 6(16) = 72 + 12 + 96 = 180. Dividing by the empirical mass: 180 / 30 = 6, confirming that the molecular formula is exactly 6 times the empirical formula.

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Le Chatelier's principle states that if a system at equilibrium is subjected to a change in conditions, the equilibrium shifts in the direction that tends to counteract that change. For pressure changes, the key factor is the difference in the total number of moles of gas on each side of the equation.

If there is no change in volume during a gaseous reaction, this means the total number of moles of gaseous products equals the total number of moles of gaseous reactants. In such a case, increasing or decreasing the pressure gives the system no direction in which to shift, because neither the forward nor the backward reaction would reduce the total number of gas molecules.

Therefore, a change in pressure will have no effect on the equilibrium position when the reaction involves equal moles of gas on both sides.

For example, in the reaction:

\[ \text{H}_2(g) + \text{I}_2(g) \rightleftharpoons 2\text{HI}(g) \]

there are 2 moles of gas on each side, so pressure changes do not shift the equilibrium.

Pressure only affects equilibrium when there is an unequal number of moles of gas on either side. In those cases, increasing pressure favours the side with fewer moles of gas, and decreasing pressure favours the side with more moles.

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The expression shown is V \(\propto\) nT/P. This can be derived from the ideal gas equation PV = nRT, which rearranges to V = nRT/P. Since R is a constant, V is directly proportional to nT/P.

This expression combines three individual gas laws into one:

• Boyle's law: V \(\propto\) 1/P (at constant n and T)
• Charles' law: V \(\propto\) T (at constant n and P)
• Avogadro's law: V \(\propto\) n (at constant T and P)

Because the expression accounts for changes in all three variables (amount of substance n, temperature T, and pressure P) simultaneously, it represents the general gas law, not any single individual law.

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The spontaneity of a reaction is determined by the Gibbs free energy change, given by:

\[\Delta G = \Delta H - T\Delta S\]

A reaction is spontaneous when \(\Delta G\) is negative.

In this question:

• \(\Delta H\) is positive and small (slightly endothermic).
• \(\Delta S\) is positive and large (significant increase in disorder).

Substituting into the equation:

\[\Delta G = (\text{small positive}) - T \times (\text{large positive})\]

Since \(T\) (absolute temperature in Kelvin) is always positive, the term \(T\Delta S\) will be a large positive number. Subtracting this large positive value from a small positive \(\Delta H\) gives:

\[\Delta G = \text{small positive} - \text{large positive} = \text{negative}\]

A negative \(\Delta G\) means the reaction is spontaneous.

Exam tip: When \(\Delta H\) is positive but \(\Delta S\) is also positive and large, the entropy term dominates, and the reaction is spontaneous, especially at higher temperatures. This is called an entropy-driven reaction.

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Brine is a concentrated solution of sodium chloride (NaCl) in water. During electrolysis of brine, the ions present are Na⁺, Cl^-, H⁺ (from water), and OH^- (from water).

At the cathode (negative electrode), reduction takes place. The two cations competing for discharge are Na⁺ and H⁺. Because hydrogen ions are much easier to reduce than sodium ions (sodium has a very negative standard electrode potential), H⁺ ions are preferentially discharged:

\[2\text{H}^+(aq) + 2e^- \rightarrow \text{H}_2(g)\]

The gas produced at the cathode is therefore hydrogen.

At the anode (positive electrode), chloride ions are oxidised to produce chlorine gas. Sodium hydroxide remains in solution. Steam is not produced during electrolysis, and oxygen would only appear at the anode if a dilute solution were used instead of concentrated brine.

Exam tip: In electrolysis of brine, remember the three products: hydrogen at the cathode, chlorine at the anode, and sodium hydroxide in solution.

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The reaction is:

\[\text{NH}_3(g) + \text{HCl}(g) \rightarrow \text{NH}_4\text{Cl}(s)\]

On the reactant side, there are 2 moles of gas (1 mole of NH₃ + 1 mole of HCl). On the product side, NH₄Cl is a solid, so there are effectively 0 moles of gas.

According to Le Chatelier's principle, when the pressure of a gaseous system at equilibrium is increased, the equilibrium shifts towards the side with fewer moles of gas to reduce the pressure.

Since the product side has fewer gaseous moles than the reactant side, increasing the pressure will shift the equilibrium to the right, favouring the product (NH₄Cl).

Note that changing pressure shifts the position of equilibrium but does not change the equilibrium constant (K). The equilibrium constant is only affected by changes in temperature, not pressure or concentration.

Exam tip: When applying Le Chatelier's principle to pressure changes, count only the moles of gaseous species on each side. Solids and liquids are not affected by pressure changes.

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An alkanoic acid (carboxylic acid) has the general formula \(\text{C}_n\text{H}_{2n}\text{O}_2\). The task is to find the molecular formula whose empirical formula is CH\(_2\)O and whose molecular formula fits the alkanoic acid pattern.

First, calculate the empirical formula mass of CH\(_2\)O:

\[12 + 2(1) + 16 = 30\]

The molecular formula must be a whole-number multiple of the empirical formula: \((\text{CH}_2\text{O})_n\). To satisfy the alkanoic acid general formula \(\text{C}_n\text{H}_{2n}\text{O}_2\), we need 2 oxygen atoms, which requires \(n = 2\):

\[(\text{CH}_2\text{O})_2 = \text{C}_2\text{H}_4\text{O}_2\]

This is ethanoic acid (acetic acid, CH\(_3\)COOH), a well-known alkanoic acid.

The relative molecular mass is:

\[2(12) + 4(1) + 2(16) = 24 + 4 + 32 = 60\]

The answer is 60.

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Ammonium trioxonitrate(V) is the IUPAC name for ammonium nitrate, NH₄NO₃. When heated gently (thermal decomposition), it breaks down as follows:

\[\text{NH}_4\text{NO}_3 \xrightarrow{\text{heat}} \text{N}_2\text{O} + 2\text{H}_2\text{O}\]

The products are dinitrogen monoxide (N₂O, also known as nitrous oxide or laughing gas) and water (H₂O).

To verify, check that the equation is balanced:

• N: 2 on each side (1 from NH₄⁺ and 1 from NO₃^- on the left; 2 in N₂O on the right).
• H: 4 on each side (4 in NH₄⁺ on the left; 4 in 2H₂O on the right).
• O: 3 on each side (3 in NO₃^- on the left; 1 in N₂O + 2 in 2H₂O on the right).

The other options are incorrect: NO₂ and H₂O would not balance correctly with the given reactant; N₂O and O₂ would leave hydrogen unaccounted for; and NO₃ is not a stable molecular product of thermal decomposition.

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Surface tension is the property of a liquid that causes its surface to behave like a stretched elastic membrane. It arises because molecules at the surface of a liquid experience a net inward pull from neighbouring molecules below and beside them, but not from above. This inward force causes the surface to contract to the smallest possible area.

For a given volume of liquid, the shape with the smallest surface area is a sphere. Therefore, when water forms small droplets (such as raindrops or drops on a waxy surface), surface tension pulls the water into a spherical shape.

The other properties do not explain the spherical shape:

• Polarity explains why water molecules attract each other (through hydrogen bonding), which contributes to surface tension, but polarity itself does not directly cause the spherical shape.
• Viscosity is the resistance of a liquid to flow. It affects how quickly water moves, not the shape of its droplets.
• Density is mass per unit volume and determines whether a substance sinks or floats, not the shape of droplets.

Exam tip: Surface tension explains several everyday observations: water forming spherical drops, insects walking on water surfaces, and a needle floating when placed gently on water.

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Solubility is defined as the mass of solute that dissolves in 100 g of solvent (water) to form a saturated solution at a given temperature.

From the question, the mass of the saturated solution is 12.5 g and the mass of anhydrous salt recovered after evaporation is 2 g. The mass of water in the solution is therefore:

\[\text{Mass of water} = 12.5 - 2 = 10.5 \text{ g}\]

Solubility is calculated as:

\[\text{Solubility} = \frac{\text{Mass of solute}}{\text{Mass of solvent}} \times 100\]

\[\text{Solubility} = \frac{2}{10.5} \times 100 = 19.05 \text{ g per 100 g of water}\]

The calculated value of 19.05 g/100 g water is closest to 19.05, which rounds to approximately 19 g/100 g. Among the available options, 19.05 does not match any value exactly. However, if the question intends the mass of water to be taken as 10 g (a common simplification in some exam settings where the saturated solution mass is approximated), the calculation becomes:

\[\text{Solubility} = \frac{2}{10} \times 100 = 20.0 \text{ g per 100 g of water}\]

The intended answer is therefore 19.05 g/100 g water by strict calculation, but the closest provided value is 20.0 g per 100 g of water.

Exam tip: Always identify the mass of solute and the mass of solvent separately from the total solution mass before applying the solubility formula.

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The equation shown is:

2XY₃(g) ⇌ X₂(g) + 3Y₂(g)

The equilibrium constant \(K_c\) is defined as the ratio of the product concentrations raised to their stoichiometric coefficients divided by the reactant concentrations raised to their stoichiometric coefficients.

From the balanced equation, the products are X₂ (coefficient 1) and Y₂ (coefficient 3), while the reactant is XY₃ (coefficient 2). Therefore:

\[K_c = \frac{[X_2][Y_2]^3}{[XY_3]^2}\]

The coefficients become exponents in the equilibrium expression, not multipliers placed in front of the concentration brackets. This is a fundamental distinction: writing \([2XY_3]\) or \([3Y_2]\) treats the coefficient as part of the concentration term, which is incorrect.

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Copper is deposited from CuSO₄ solution by the reduction of Cu²⁺ ions:

\[\text{Cu}^{2+} + 2e^- \rightarrow \text{Cu}\]

This means each mole of copper requires 2 moles of electrons (2 faradays) to be deposited.

Step 1: Calculate the moles of copper to be deposited.

\[n_{\text{Cu}} = \frac{\text{mass}}{\text{molar mass}} = \frac{4.5}{64} = 0.0703125 \text{ mol}\]

Step 2: Calculate the total charge required.

Since 1 mole of Cu requires 2 faradays:

\[Q = n_{\text{Cu}} \times 2 \times F = 0.0703125 \times 2 \times 96500\]

\[Q = 0.140625 \times 96500 = 13570.3 \text{ C}\]

Step 3: Calculate the time using \(Q = It\).

\[t = \frac{Q}{I} = \frac{13570.3}{2.5} = 5428 \text{ sec}\]

The time required is 5428 seconds.

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The equation shows ethanol (C₂H₅OH) reacting with ethanoic acid (CH₃COOH) to form ethyl ethanoate (CH₃COOC₂H₅) and water (H₂O).

This is an esterification reaction. Esterification is the reaction between a carboxylic acid and an alcohol to produce an ester and water. It is typically catalysed by a concentrated strong acid such as tetraoxosulphate(VI) acid (H₂SO₄), and the reaction is reversible, indicated by the equilibrium sign (⇌).

The general equation is:

\[\text{Carboxylic acid} + \text{Alcohol} \xrightleftharpoons{\text{H}_2\text{SO}_4} \text{Ester} + \text{Water}\]

The other options do not apply here:

• Condensation is a broader term for reactions where two molecules combine with the loss of a small molecule (often water). While esterification is technically a type of condensation, the specific and more accurate name for this reaction between an acid and an alcohol is esterification.
• Fermentation is the enzymatic conversion of sugars (such as glucose) to ethanol and carbon(IV) oxide.
• Hydrolysis is the reverse of esterification, where an ester is broken down by water (or alkali) into the parent acid and alcohol.

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Carbon(II) oxide is the IUPAC-style name for carbon monoxide (CO). It is a colourless, odourless, and highly toxic gas that is a major air pollutant, especially in urban areas.

The primary source of carbon monoxide as an air pollutant is the incomplete combustion of carbon-containing fuels. When fuels such as petrol, diesel, kerosene, coal, or wood burn with an insufficient supply of oxygen, carbon is only partially oxidised to CO instead of fully oxidised to CO₂:

\[ 2\text{C} + \text{O}_2 \rightarrow 2\text{CO} \]

Vehicle exhaust emissions are the single largest contributor of CO to the atmosphere, along with industrial furnaces and domestic cooking fires that operate under oxygen-poor conditions.

Respiration produces carbon dioxide (CO₂), not carbon monoxide. Photochemical smog is a secondary pollution phenomenon caused by sunlight acting on nitrogen oxides and volatile organic compounds; it is not a source of CO itself. Decomposition of sewage releases gases such as methane (CH₄) and hydrogen sulphide (H₂S), not carbon monoxide.

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The diagram shows the equation 2NaCl(l) → 2Na(l) + Cl₂(g) with electricity as the energy source. This is the electrolysis of molten sodium chloride to produce metallic sodium and chlorine gas.

This industrial process is known as the Downs process, named after J.C. Downs who patented the Downs cell in 1924. In the Downs cell, molten NaCl (often mixed with CaCl₂ to lower the melting point from 801°C to about 600°C) is electrolysed. At the cathode, Na⁺ ions are reduced to liquid sodium metal, while at the anode, Cl⁻ ions are oxidised to produce chlorine gas.

The Bosch process produces hydrogen gas from water gas. The Chlor-alkali process electrolyses aqueous (not molten) NaCl to give NaOH, Cl₂, and H₂. The Browning process is not a standard industrial chemistry term in this context.

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Freons are a group of chlorofluorocarbons (CFCs) - synthetic compounds containing chlorine, fluorine, and carbon. They were widely used as propellants in aerosol cans, as refrigerants in air conditioners and refrigerators, and as solvents in industrial cleaning.

When released into the atmosphere from these sources, freons rise to the stratosphere where ultraviolet radiation breaks them down, releasing chlorine atoms. These chlorine atoms catalytically destroy ozone molecules, contributing to the depletion of the ozone layer.

Fossil fuel combustion releases carbon dioxide, sulphur dioxide, and nitrogen oxides, but not freons. Photosynthesis is a biological process that produces oxygen and consumes carbon dioxide. Organic decay releases methane and carbon dioxide. None of these processes involve freons.

The Montreal Protocol (1987) restricted the production and use of CFCs, leading to a gradual recovery of the ozone layer.

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Producer gas is a mixture of carbon monoxide (CO) and nitrogen (N₂), while water gas is a mixture of carbon monoxide (CO) and hydrogen (H₂). Both gases contain CO, which is toxic and must be removed for certain industrial applications.

Ammoniacal copper(I) chloride solution (CuCl dissolved in ammonia) is the reagent used to selectively absorb CO from these gas mixtures. The CO molecules form a coordination complex with the copper(I) ions in solution:

\[ \text{CuCl} + \text{CO} + 2\text{NH}_3 \rightarrow [\text{Cu(CO)(NH}_3\text{)}_2]\text{Cl} \]

This reaction is reversible: gentle heating releases the absorbed CO and regenerates the ammoniacal CuCl solution for reuse.

The other gases in these mixtures are not absorbed by this reagent. Hydrogen (H₂) does not form stable complexes with Cu⁺ under these conditions. Nitrogen (N₂) is chemically inert at room temperature. Carbon dioxide (CO₂) would be absorbed by alkaline solutions such as NaOH or KOH, not by ammoniacal CuCl.

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The corrosion of iron is specifically called rusting. Rusting occurs when iron reacts with oxygen and water (moisture) over time to form hydrated iron(III) oxide, commonly known as rust:

\[4\text{Fe} + 3\text{O}_2 + 6\text{H}_2\text{O} \rightarrow 4\text{Fe(OH)}_3\]

The iron(III) hydroxide gradually dehydrates to form the familiar reddish-brown rust (Fe₂O₃ . xH₂O). Both oxygen and water must be present for rusting to occur; iron does not rust in dry air or in air-free water.

The other options are different processes entirely: burning (combustion) is a rapid reaction with oxygen involving flame and heat; galvanizing is a method of preventing corrosion by coating iron with a layer of zinc; alloying is mixing metals together to form an alloy (such as stainless steel), which is also a corrosion-prevention strategy, not a corrosion process.

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An alkaline gas is a gas that dissolves in water to produce a solution with a pH greater than 7 (a basic solution).

NH₃ (ammonia) is the classic example. When ammonia dissolves in water, it reacts to form ammonium hydroxide:

\[\text{NH}_3(g) + \text{H}_2\text{O}(l) \rightleftharpoons \text{NH}_4^+(aq) + \text{OH}^-(aq)\]

The production of hydroxide ions (OH^-) makes the solution alkaline.

The other gases are not alkaline:

• HCl (hydrogen chloride) dissolves in water to form hydrochloric acid, which is strongly acidic.
• N₂ (nitrogen) is an inert, unreactive gas that does not dissolve appreciably in water and produces neither acidic nor basic solutions.
• NO₂ (nitrogen dioxide) dissolves in water to form a mixture of nitric acid and nitrous acid, both of which are acidic.

Exam tip: Ammonia is the only common alkaline gas encountered at this level. Its characteristic pungent smell and ability to turn moist red litmus paper blue are standard identification tests.

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A diagonal relationship in the periodic table refers to the similarity in properties between an element in Period 2 and the element diagonally below and to its right in Period 3. This occurs because moving one period down increases size and metallic character, while moving one group to the right decreases them, so the two effects partially cancel out.

The well-established diagonal pairs are:

• Li (Period 2, Group 1) and Mg (Period 3, Group 2)
• Be (Period 2, Group 2) and Al (Period 3, Group 13)
• B (Period 2, Group 13) and Si (Period 3, Group 14)

From the given options, B and Si is one of the classic diagonal pairs. They share similar properties such as forming covalent compounds, acting as semiconductors, and forming acidic oxides.

The other options are not diagonal pairs:

• Be and Si are two groups apart diagonally, not one.
• Na and Be are in adjacent periods but Na is below and to the left of Be, not diagonal in the correct direction.
• Li and Al are separated by two groups in the diagonal direction.

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The equation is:

\[2\text{X} + 2\text{HCl} \rightarrow 2\text{XCl} + \text{H}_2\]

The product formed is XCl, which tells us that element X combines with chlorine in a 1:1 ratio. This means X has a valency of +1 and forms a monovalent chloride.

Examining the options:

• K (potassium): Group 1 element with a valency of +1. It forms KCl. The equation becomes \(2\text{K} + 2\text{HCl} \rightarrow 2\text{KCl} + \text{H}_2\), which balances correctly.
• Mg (magnesium): Group 2 element with a valency of +2. It forms MgCl₂, not MgCl. Does not match.
• Ca (calcium): Group 2 element with a valency of +2. It forms CaCl₂, not CaCl. Does not match.
• Ba (barium): Group 2 element with a valency of +2. It forms BaCl₂, not BaCl. Does not match.

Only potassium (K) has a valency of +1 and forms a chloride with the formula XCl, making it the correct identity of X.

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The homologous series of alkenes are unsaturated hydrocarbons that contain exactly one carbon-carbon double bond (C=C). Their general molecular formula is \(\text{C}_n\text{H}_{2n}\), where n is the number of carbon atoms (n >= 2).

To verify, consider a few members:

• Ethene (n = 2): C₂H₄ = C₂H₂₍₂₎
• Propene (n = 3): C₃H₆ = C₃H₂₍₃₎
• Butene (n = 4): C₄H₈ = C₄H₂₍₄₎

Each formula fits \(\text{C}_n\text{H}_{2n}\).

The other general formulae belong to different homologous series: \(\text{C}_n\text{H}_{2n+2}\) represents alkanes (saturated hydrocarbons), \(\text{C}_n\text{H}_{2n-2}\) represents alkynes (with a triple bond), and \(\text{C}_n\text{H}_{2n+1}\text{OH}\) represents alkanols (alcohols).

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According to Graham's law of diffusion, the rate of diffusion of a gas is inversely proportional to the square root of its molar mass:

\[\text{Rate} \propto \frac{1}{\sqrt{M}}\]

This means lighter gases diffuse faster and heavier gases diffuse slower. To arrange gases in order of increasing rate of diffusion, we arrange them from heaviest (slowest) to lightest (fastest).

Calculate the molar masses of the gases that appear in the options:

Arranging from heaviest to lightest (i.e., increasing rate of diffusion):

SO₂ (64) → O₂ (32) → NH₃ (17) → H₂ (2)

This matches the sequence SO₂, O₂, NH₃, H₂. The heaviest gas (SO₂) diffuses most slowly, and the lightest gas (H₂) diffuses most rapidly.

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Tetraoxosulphate(VI) acid is the IUPAC name for sulphuric acid, \(\text{H}_2\text{SO}_4\). Its large-scale industrial manufacture uses the Contact process.

The Contact process involves three main stages:

1. Sulphur is burned in air to produce sulphur(IV) oxide: \(\text{S} + \text{O}_2 \rightarrow \text{SO}_2\)
2. Sulphur(IV) oxide is oxidised to sulphur(VI) oxide using a vanadium(V) oxide (\(\text{V}_2\text{O}_5\)) catalyst at about 450 \(^\circ\text{C}\): \(2\text{SO}_2 + \text{O}_2 \rightleftharpoons 2\text{SO}_3\)
3. The \(\text{SO}_3\) is absorbed in concentrated \(\text{H}_2\text{SO}_4\) to form oleum (\(\text{H}_2\text{S}_2\text{O}_7\)), which is then carefully diluted with water to give concentrated sulphuric acid.

The other named processes serve different purposes. The Haber process manufactures ammonia from nitrogen and hydrogen. The Frasch process is used for mining sulphur deposits underground using superheated water. The Bosch process (or water-gas shift reaction) produces hydrogen from carbon monoxide and steam. None of these produces sulphuric acid.

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A metal is said to occur in the free state (or native state) in nature when it is found as the pure, uncombined element rather than in a compound (such as an ore). This happens only for metals that are very low in the reactivity series, meaning they are resistant to reaction with oxygen, water, and acids.

Silver (Ag) is one such metal. It is found as native silver in the earth's crust because of its very low chemical reactivity. Gold and platinum are other classic examples of metals found in the free state.

Zinc and iron are too reactive to exist as free metals in nature. They readily react with oxygen and moisture to form oxides and other compounds, so they are always found as ores (e.g., zinc as zinc blende ZnS, iron as haematite Fe₂O₃). While copper can occasionally be found native, silver is the stronger answer here because it is less reactive than copper and more commonly occurs in the uncombined state.

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Dilute nitric acid (HNO\(_3\)) is an oxidising acid, which means it usually oxidises the metal and is itself reduced to nitrogen oxides (such as NO or NO\(_2\)) rather than producing hydrogen gas. This is different from non-oxidising acids like dilute HCl or dilute H\(_2\)SO\(_4\), which readily liberate H\(_2\) with reactive metals.

However, magnesium (Mg) is an exception. Because magnesium is extremely reactive (high up in the electrochemical series), it reacts so vigorously with very dilute HNO\(_3\) that the reaction proceeds faster than the acid can act as an oxidising agent. The result is that hydrogen gas is liberated:

\[\text{Mg} + 2\text{HNO}_3\text{(very dilute)} \rightarrow \text{Mg(NO}_3\text{)}_2 + \text{H}_2\uparrow\]

Copper (Cu) is below hydrogen in the activity series and cannot displace hydrogen from any acid under normal conditions. Zinc (Zn) reacts with dilute HNO\(_3\) but produces NO gas rather than H\(_2\), because it is not reactive enough to overcome the oxidising nature of the acid. Calcium (Ca) is very reactive but reacts explosively with water itself and, in practice with dilute HNO\(_3\), produces nitrogen oxides or ammonia rather than clean H\(_2\) liberation; the standard examination answer for this question is magnesium.

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In qualitative analysis, ions are classified as either acid radicals (anions) or basic radicals (cations).

• Acid radicals are the negative ions (anions) derived from acids. Examples include CO₃^2- (from H₂CO₃), SO₄^2- (from H₂SO₄), NO₃^- (from HNO₃), PO₄^3- (from H₃PO₄), and Cl^- (from HCl).
• Basic radicals are the positive ions (cations) derived from bases. Examples include Na⁺, K⁺, Mg²⁺, Ca²⁺, and Fe²⁺.

The question asks which group contains only acid radicals. Examining each option:

• CO₃^2-, PO₄^3-, Mg²⁺: contains Mg²⁺, which is a basic radical (cation).
• CO₃^2-, SO₄^2-, Na⁺: contains Na⁺, which is a basic radical (cation).
• CO₃^2-, SO₄^2-, NO₃^-: all three are anions derived from acids. This group contains only acid radicals.
• CO₃^2-, SO₄^2-, K⁺: contains K⁺, which is a basic radical (cation).

The correct answer is the group containing CO₃^2-, SO₄^2-, and NO₃^-, as all three are acid radicals.