WAEC SSCE - Chemistry - 2002

If two metallic ions of the same concentration migrate to a graphite cathode, the one that would be preferentially discharged is the one that has the

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When two metallic ions of the same concentration migrate to a graphite cathode, the ion that would be preferentially discharged is the one that has a stronger tendency to accept electrons. This is because during the process of electrolysis, the positively charged ions are reduced (gain electrons) at the cathode. The ion with a stronger tendency to accept electrons is more likely to be reduced at the cathode than the other ion, leading to its preferential discharge. This is known as the selective discharge of ions. The mass, electrical charge, and stability of the ion in solution do not affect its preferential discharge.

Which of the following compounds can be represented by the molecular formula C₂H₆O?

Detalles de la respuesta

The molecular formula C₂H₆O indicates that the compound contains 2 carbon atoms, 6 hydrogen atoms, and 1 oxygen atom. Among the options given, the only compound that fits this molecular formula is Ethanol (C₂H₆O), which has the chemical formula CH₃CH₂OH. Propanol (Propanol), has the molecular formula C₃H₈O, Methanoic acid (Formic acid) has the molecular formula HCOOH and Glucose has the molecular formula C₆H₁₂O₆.

Calculate the mass of sodium hydroxide in 5.00 dm³ of a 0.125 mol dm^-3 solution. [NaOH = 40 g mol^-1]

Detalles de la respuesta

To calculate the mass of sodium hydroxide in 5.00 dm³ of a 0.125 mol dm^-3 solution, we need to use the formula: mass = moles × molar mass First, let's calculate the number of moles of NaOH in 5.00 dm³ of the solution: moles = concentration × volume moles = 0.125 mol dm^-3 × 5.00 dm³ moles = 0.625 mol Now, we can use the formula to calculate the mass of NaOH: mass = 0.625 mol × 40 g mol^-1 mass = 25.0 g Therefore, the mass of sodium hydroxide in 5.00 dm³ of a 0.125 mol dm^-3 solution is 25.0 g. Answer: 25.0 g

Which of the following decreases when a given mass of gas is compressed to half its initial volume?

Detalles de la respuesta

When a given mass of gas is compressed to half its initial volume, the volume of the container holding the gas is reduced, but the number of gas molecules remains constant. This means that the gas molecules have less space to move around, and the average intermolecular distance between them decreases. As the distance between the molecules decreases, the frequency of collisions between them increases. Thus, the correct answer is: - Average intermolecular distance

Which of the following salt solutions will have a pH greater than 7?

Detalles de la respuesta

Among the given options, Na₂CO_3(aq) is the only salt solution that can have a pH greater than 7. When Na₂CO₃ is dissolved in water, it undergoes hydrolysis to produce NaOH and HCO₃^- ions. The hydroxide ion concentration increases, which makes the solution basic. Hence, the solution will have a pH greater than 7. On the other hand, NaCl, Na₂SO₄, and NaHSO₄ are all salts of strong acids and strong bases, or weak acids and strong bases, and therefore their solutions will be neutral or slightly acidic but never basic.

The atomic number of chlorine is 17. What is the number of electrons in a chloride ion?

Detalles de la respuesta

Chlorine has 17 electrons because it has 17 protons, which is its atomic number. A chloride ion (Cl^-) has gained one electron to form a negative ion. Therefore, it now has one more electron than the number of protons (which is still 17). This means that a chloride ion has 18 electrons. So, the answer is (c) 18.

Which of the following types of alkanols undergo oxidation to produce alkanoic acids? 1. Primary alkanols 11. Secondary alkanols 111. Tertiary alkanols

Detalles de la respuesta

Only primary alkanols undergo oxidation to produce alkanoic acids. This is because primary alkanols have a hydrogen atom attached to the carbon atom with the -OH group, which can be easily oxidized to a carboxylic acid (-COOH) group. Secondary and tertiary alkanols lack this hydrogen atom and cannot be directly oxidized to alkanoic acids.

If the relative molecular mass of an element is not a whole number, it can be deduced that the element is

Detalles de la respuesta

If the relative molecular mass of an element is not a whole number, it can be deduced that the element is an isotopic mixture. This is because the relative molecular mass of an element is the average of the masses of all the isotopes of that element, taking into account their relative abundances. Isotopic mixture means that the element is composed of different isotopes with different masses and different relative abundances. For example, chlorine has two isotopes, Cl-35 and Cl-37, which have relative abundances of 75.5% and 24.5%, respectively. The average atomic mass of chlorine is therefore not a whole number, but a weighted average of the masses of these isotopes, taking into account their abundances.

Iron is often galvanized in order to

Detalles de la respuesta

Iron is often galvanized to protect it against corrosion. Galvanizing is a process of coating iron or steel with a layer of zinc to prevent rusting. The zinc acts as a sacrificial anode, which means that it corrodes before the iron or steel underneath it, providing a protective layer that prevents the underlying metal from coming into contact with moisture and oxygen, which are the main agents of corrosion. This process helps to extend the lifespan of iron or steel objects and structures.

An element Y has the electronic configuration 1s²2s²2p⁶6²3p₄. To what period does it belong in the periodic table?

Detalles de la respuesta

The electronic configuration of an element provides information about its position in the periodic table. The period of an element is determined by the highest energy level occupied by electrons in the ground state. In the given electronic configuration, the highest energy level occupied by electrons is the third energy level. Therefore, element Y belongs to period 3 of the periodic table.

One of the methods used for combating air pollution is

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Legislation against some industrial practices is one of the methods used for combating air pollution. This involves enacting laws and regulations to control or limit the amount of pollutants released into the atmosphere by industries. The laws may require industries to install pollution control devices or use cleaner production methods that generate fewer pollutants. The aim is to reduce the amount of pollutants in the air, thus reducing air pollution and its negative impacts on the environment and human health.

Which of the following processes takes place during the production of margarine from vegetable oils?

Detalles de la respuesta

The process that takes place during the production of margarine from vegetable oils is hydrogenation. Hydrogenation is a chemical process that involves the addition of hydrogen gas to unsaturated fatty acids in vegetable oils, resulting in the conversion of the unsaturated fatty acids into saturated fatty acids. This process makes the vegetable oils more solid and stable, improving their texture and shelf life. The other options listed are not relevant to the production of margarine from vegetable oils. Esterification involves the formation of esters from carboxylic acids and alcohols, hydrolysis involves the breakdown of molecules into their constituent parts by the addition of water, and saponification involves the reaction of a fat or oil with a strong base to produce soap and glycerol.

What will happen if more heat is applied to the following system at equilibrium? X2(g) + 3Y 2(g) ⇌ 2XY3(g); ∆H = xKJmol-1

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If two metallic ions of the same concentration migrate to a graphite cathode, the one that would be preferentially discharged is the one that has the

Detalles de la respuesta

In an electrolytic cell, when two metallic ions of the same concentration migrate to a graphite cathode, the one that would be preferentially discharged is the one that has the stronger tendency to accept electrons. This is because reduction, which involves the gain of electrons, occurs at the cathode. The ion with a stronger tendency to accept electrons will have a higher reduction potential and will therefore be preferentially reduced at the cathode.

The energy value of petrol can be determined by a

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If a metal R forms a compound with the formula R(NO₃)² , the formula of its tetraoxosulphate (IV) is

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Which of the following compounds can be represented by the molecular formula C₂H₆O?

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The molecular formula C₂H₆O indicates that the compound has two carbon atoms, six hydrogen atoms, and one oxygen atom. By checking the options: - Propanal has the molecular formula C₃H₆O, which means it has three carbon atoms and cannot be represented by the given formula. - Ethanol has the molecular formula C₂H₆O and can be represented by the given formula. - Methanoic acid has the molecular formula CH₂O₂, which means it has only one carbon atom and cannot be represented by the given formula. - Glucose has the molecular formula C₆H₁₂O₆, which means it has six carbon atoms and cannot be represented by the given formula. Therefore, the compound that can be represented by the molecular formula C₂H₆O is ethanol.

The reaction between ethene and chlorine to form 1, 2-dichloroethane is

Detalles de la respuesta

The reaction between ethene and chlorine to form 1, 2-dichloroethane is an addition reaction. In this reaction, the double bond between the two carbon atoms in ethene is broken, and each carbon atom bonds to a chlorine atom to form 1,2-dichloroethane. The reaction is called an addition reaction because the two molecules add together to form a single molecule, rather than substituting or removing atoms as in a substitution or oxidation reaction.

Consider the following equation: Cr₂O^2-_7(aq) + 14H⁺_(aq) + 6e^- → 2Cr³⁺_(ag) + 7H₂O₍₁₎. The oxidation number of chromium changes from

Detalles de la respuesta

The oxidation number is a number that represents the charge of an atom in a compound. In the given equation, the chromium atom changes from a +6 oxidation state on the reactant side (Cr₂O^2-_(aq)) to a +3 oxidation state on the product side (2Cr³⁺_(aq)). This means that the oxidation number of chromium has decreased by 3, indicating that it has undergone reduction. Therefore, the correct answer is +6 to +3.

The following compounds are hydrocarbons except

Detalles de la respuesta

The compound that is not a hydrocarbon is methylpropanoate. A hydrocarbon is a compound that contains only carbon and hydrogen atoms. - 2-methylbutane is a hydrocarbon because it only contains carbon and hydrogen atoms. - Benzene is a hydrocarbon with a ring structure made up of carbon atoms and hydrogen atoms. - Cyclohexane is also a hydrocarbon with a ring structure made up of carbon atoms and hydrogen atoms. Methylpropanoate, on the other hand, contains carbon, hydrogen, and oxygen atoms, which means it is not a hydrocarbon.

Calculate the mass of chlorine gas which occupies a volume of 1.12 dm\(^3\) a volume of 1.12 dm\(^3\) at s.t.p [Cl = 35.5, 1 mole og gas occupies 22.4 dm\(^3\) at s. t. p ]

Detalles de la respuesta

The given information includes the volume of chlorine gas and the conditions of temperature and pressure (s.t.p). The formula for calculating the mass of a gas at s.t.p is: mass = (volume of gas at s.t.p in dm³) x (molar mass of gas) / (molar volume at s.t.p) The molar mass of chlorine gas (Cl₂) is 2 x atomic mass of Cl = 2 x 35.5 = 71 g/mol. The molar volume of any gas at s.t.p is 22.4 dm³. Plugging in the values, we get: mass = (1.12 dm³) x (71 g/mol) / (22.4 dm³/mol) = 3.55 g Therefore, the mass of chlorine gas which occupies a volume of 1.12 dm³ at s.t.p is 3.55 g. The correct option is (b) 3.55g.

CuSO₄5H₂O can be obtained from an aqueous solution of copper (ll) tetraoxosulphate (VI) by

Detalles de la respuesta

CuSO₄5H₂O can be obtained from an aqueous solution of copper (II) tetraoxosulphate (VI) by **crystallization**. Crystallization is the process of separating a solid substance from a liquid by cooling the liquid or allowing it to evaporate slowly. In this case, by slowly evaporating the water from the solution of copper (II) tetraoxosulphate (VI), CuSO₄5H₂O will crystallize out of the solution as blue crystals. The crystals obtained will have a definite shape and size, and can be filtered and dried to obtain the pure solid CuSO₄5H₂O.

A major factor considered in selecting a suitable method for preparing a simple salt is its

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One major factor considered in selecting a suitable method for preparing a simple salt is its solubility in water. The method chosen should be able to effectively dissolve the necessary reactants in water to form the salt. If the salt is insoluble in water, a different method such as precipitation or metathesis may be used. Additionally, the solubility of the salt can affect the purification process and the yield of the final product. Therefore, the solubility of the salt in water is an important consideration in selecting a suitable method for preparing it.

A visible change is observed when a strip of iron is placed in an aqueous solution of

Detalles de la respuesta

When a strip of iron is placed in an aqueous solution of copper sulfate (CuSO₄), a visible change is observed. The iron strip reacts with the copper ions in the solution and displaces them, forming iron sulfate (FeSO₄) and copper metal. This reaction is known as a single displacement or replacement reaction. The copper metal formed will appear as a reddish-brown solid on the iron strip, causing a visible change.

What is the expression for the equilibrium constant (K_c) for the following reaction? N_2(g) + O_2(g) ⇌ 2NO_(g)

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Which of the following ions require the largest quantity of electricity for discharge at an electrode?

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The atom of an element X is represented as \(^y_z X\). The basic chemical properties of X depend on the value of

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In the symbol of an element, the number of protons (Z) in its nucleus is called its atomic number. The atomic number determines the element's identity and its basic chemical properties. Elements with the same atomic number have the same number of protons, so they have the same electronic configuration, meaning they have similar chemical behavior. Therefore, the value of Z in the symbol \(^y_z X\) determines the element's basic chemical properties.

What volume of hydrogen will be produced if 100cm³ of ammonia is completely decomposed at constant temperature and pressure? The equation for reaction is 2NH_3(g) → N_2(g) + 3H_2(g)

Detalles de la respuesta

The balanced chemical equation for the reaction between ammonia and hydrogen is: 2NH_3(g) → N_2(g) + 3H_2(g) From the equation, we can see that 2 moles of ammonia reacts to produce 3 moles of hydrogen. Therefore, the ratio of ammonia to hydrogen is 2:3. This means that for every 2 moles of ammonia, 3 moles of hydrogen will be produced. To determine the volume of hydrogen produced, we need to use the volume of ammonia given in the question. We can assume that the ammonia is measured under the same temperature and pressure as the hydrogen produced. Since we are given the volume of ammonia, we need to convert it to moles using the ideal gas law: PV = nRT Where: P = pressure (constant) V = volume of gas (in liters) n = number of moles R = ideal gas constant T = temperature (constant) Assuming constant temperature and pressure, we can simplify the equation to: V = n Therefore, the volume of ammonia (100 cm³) is equal to its number of moles. Using the ratio from the balanced equation, we can calculate the number of moles of hydrogen produced: 2 moles NH₃ : 3 moles H₂ 100 cm³ NH₃ : x cm³ H₂ x = (100 cm³ NH₃)(3 mol H₂/2 mol NH₃) = 150 cm³ H₂ Therefore, the volume of hydrogen produced is 150 cm³. The correct answer is (B) 150 cm³.

A hydrogen atom which has lost an electron contains

Detalles de la respuesta

A hydrogen atom which has lost an electron contains one proton only. When an electron is lost from a neutral hydrogen atom, the positively charged proton remains in the nucleus while the electron is lost to the surrounding environment. This results in a positively charged hydrogen ion, also known as a proton. Therefore, the correct answer is "one proton only".

When alkynes are hydrogenated completely they produce compounds with the general molecular formula

Detalles de la respuesta

When alkynes are hydrogenated completely, they produce compounds with the general molecular formula C_nH_2n+2. This is because the double or triple bond between the carbon atoms in the alkyne molecule is converted to a single bond by the addition of hydrogen atoms. As a result, each carbon atom in the original alkyne molecule becomes bonded to two hydrogen atoms, leading to the formula C_nH_2n+2 for the fully hydrogenated compound.

A suitable reagent for distinguishing between ethanoic acid ethanol is

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The gas given off when NH₄ Cl is heated with an alkali is

Detalles de la respuesta

When NH₄Cl is heated with an alkali such as NaOH, it undergoes a decomposition reaction to form NH₃ gas, water, and a salt. The balanced chemical equation for the reaction is: NH₄Cl + NaOH → NH₃ + H₂O + NaCl Therefore, the gas given off is NH₃, which is ammonia (option D).

Which of the following statements about soapless detergents is correct? They

Detalles de la respuesta

Soapless detergents, also known as synthetic detergents, are different from soaps in that they do not contain fatty acids as their active ingredient. Rather, they contain synthetic surfactants that are more soluble in water and are better at removing dirt and oils. Of the given options, the correct statement is that soapless detergents lather readily with hard water. This is because hard water contains dissolved metal ions, such as calcium and magnesium, which can react with the soap molecules and form an insoluble precipitate, reducing its cleaning ability. However, since synthetic detergents do not contain the same fatty acid molecules as soaps, they do not form this precipitate and can therefore lather well even in hard water.

Determine the mass of sulphur (IV) oxide obtained when 91.0g of oxygen reacts completely with sulphur according to the following equation: S_(s) + O_2(g) → SO_2(g) [S = 32; O = 16]

Detalles de la respuesta

The balanced chemical equation for the reaction between sulfur and oxygen to form sulfur dioxide is: S + O₂ → SO₂ This equation tells us that 1 mole of sulfur reacts with 1 mole of oxygen to produce 1 mole of sulfur dioxide. We are given the mass of oxygen that reacts completely with sulfur, which is 91.0 g. We can use this information to calculate the number of moles of oxygen: Number of moles of oxygen = Mass of oxygen / Molar mass of oxygen Number of moles of oxygen = 91.0 g / 32 g/mol Number of moles of oxygen = 2.84 mol From the balanced equation, we can see that the number of moles of sulfur dioxide produced is equal to the number of moles of oxygen that reacts. Therefore, we have: Number of moles of sulfur dioxide = 2.84 mol Now we can use the molar mass of sulfur dioxide to calculate its mass: Mass of sulfur dioxide = Number of moles of sulfur dioxide × Molar mass of sulfur dioxide Mass of sulfur dioxide = 2.84 mol × (32 g/mol + 2 × 16 g/mol) Mass of sulfur dioxide = 2.84 mol × 64 g/mol Mass of sulfur dioxide = 182 g Therefore, the mass of sulfur dioxide produced when 91.0 g of oxygen reacts completely with sulfur is 182 g. So, the correct option is (d) 182g.

The following compounds are hydrocarbons except

Detalles de la respuesta

Among the given options, only "methylpropanoate" is not a hydrocarbon. This is because a hydrocarbon is a compound made up of only hydrogen and carbon atoms. "Methylpropanoate," on the other hand, contains an additional oxygen atom in its structure, making it an ester compound. In contrast, "2-methylbutane," "benzene," and "cyclohexane" are all hydrocarbons as they contain only hydrogen and carbon atoms in their structures.

The atomic number of chlorine is 17. What is the number of electrons in a chloride ion?

Detalles de la respuesta

Chlorine has an atomic number of 17, which means it has 17 protons and 17 electrons in its neutral state. When chlorine gains an electron, it becomes a chloride ion with a charge of -1. The negative charge on the chloride ion indicates that it has one more electron than protons. Therefore, the number of electrons in a chloride ion is 17 + 1 = 18. Hence, the answer is 18.

The conclusion from Rutherford's alpha-scattering experiment is that

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Rutherford's alpha-scattering experiment led to the conclusion that atoms are mostly empty space with a small, dense, positively charged nucleus. This was based on the observation that when alpha particles were fired at a thin gold foil, most of the particles passed straight through the foil, but a few were deflected at large angles. This indicated that the positive charge of the atom was concentrated in a small, dense nucleus, and that most of the atom was empty space.

Which of the following statements about soapless detergents is correct? They

Detalles de la respuesta

Soapless detergents are also known as synthetic detergents. They are made up of molecules that are not affected by hard water, unlike soap molecules that react with hard water to form scum. Therefore, soapless detergents do not form a curdy precipitate with soft water. So, the correct statement is that they lather readily with hard water.

The reaction of sucrose with dilute HCI produces

Detalles de la respuesta

The reaction of sucrose with dilute HCl produces glucose and fructose. This process is known as hydrolysis, which means the breaking of a bond by the addition of water. The sucrose molecule is a disaccharide made up of one molecule of glucose and one molecule of fructose, held together by a glycosidic bond. When sucrose is hydrolyzed with dilute HCl, the glycosidic bond is broken, and glucose and fructose are formed as the products.

Which of the following elements has a ground state electronic configuration with an incomplete penultimate shell?

Detalles de la respuesta

The electronic configuration of an atom refers to the arrangement of electrons in its various shells or energy levels. The penultimate shell is the second-to-last shell of an atom. An incomplete penultimate shell means that the penultimate shell is not completely filled with electrons. Out of the given options, Iron has a ground state electronic configuration with an incomplete penultimate shell. The electronic configuration of Iron (Fe) is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶. In this configuration, the 3d sub-shell is incomplete with 6 electrons, and the 4s sub-shell is complete with 2 electrons. Therefore, Iron has an incomplete penultimate shell. Sodium, Aluminium, and Calcium all have complete penultimate shells in their ground state electronic configurations. Sodium (Na) has an electronic configuration of 1s² 2s² 2p⁶ 3s¹, Aluminium (Al) has an electronic configuration of 1s² 2s² 2p⁶ 3s² 3p¹, and Calcium (Ca) has an electronic configuration of 1s² 2s² 2p⁶ 3s² 3p⁶ 4s². In all of these configurations, the penultimate shell is complete, and thus none of these elements have an incomplete penultimate shell. Therefore, the correct answer is Iron.

Which of the following substances increases in mass when heated in air?

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Which of the following compounds will leave a metal residue when heated?

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Which of following acids forms normal salts only?

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If the relative molecular mass of an element is not a whole number, it can be deduced that the element is

Detalles de la respuesta

If the relative molecular mass of an element is not a whole number, it can be deduced that the element is an isotopic mixture. This is because the relative molecular mass is a weighted average of the masses of all the isotopes of the element, taking into account their abundance. If the element has only one isotope, its relative molecular mass will be a whole number. However, if the element has two or more isotopes with different masses and abundances, its relative molecular mass will be a decimal or fraction. Therefore, the non-whole number relative molecular mass indicates that the element is made up of a mixture of isotopes with different masses and abundances.

Which of the following processes takes place during the production of margarine from vegetable oils?

Detalles de la respuesta

The correct process that takes place during the production of margarine from vegetable oils is hydrogenation. In this process, vegetable oils, which are unsaturated, are reacted with hydrogen gas in the presence of a catalyst such as nickel. This process converts the unsaturated fatty acids in the vegetable oils into saturated fatty acids, making the oils more solid at room temperature. The hydrogenation process also increases the shelf life of the oils and gives the margarine a desirable texture and consistency. Therefore, is the correct answer.

When alkynes are hydrogenated completely, they produce compounds with the general molecular formula

Detalles de la respuesta

When alkynes are completely hydrogenated, each triple bond (-C≡C-) is converted to a single bond (-C-C-) and adds two hydrogen atoms. Therefore, each alkyne molecule loses two hydrogen atoms and gains two carbon atoms. The general molecular formula of completely hydrogenated alkynes is C _nH _2n+2. Thus, is the correct answer.

When concentrated sodium chloride solution is electrolysed using inert electrodes, the products are

Detalles de la respuesta

When a concentrated sodium chloride solution is electrolyzed using inert electrodes, the products are hydrogen gas and chlorine gas. This is because the electrolysis of the sodium chloride solution leads to the formation of chloride ions (Cl-) and hydrogen ions (H+) at the anode and cathode respectively. At the anode, chloride ions (Cl-) are oxidized to form chlorine gas (Cl2) while at the cathode, hydrogen ions (H+) are reduced to form hydrogen gas (H2). The sodium ions (Na+) do not participate in the reaction as the electrodes are inert and do not react with them.

Which of the following ions require the largest quantity of electricity for discharge at an electrode?

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When concentrated sodium chloride solution is electrolysed using inert electrodes the products are?

Detalles de la respuesta

When concentrated sodium chloride solution is electrolysed using inert electrodes, the products are hydrogen and chlorine gases. This process is known as the electrolysis of brine. At the cathode (negative electrode), hydrogen ions (H+) from water are reduced to hydrogen gas (H2). At the anode (positive electrode), chloride ions (Cl-) from sodium chloride are oxidized to form chlorine gas (Cl2). The sodium ions (Na+) and hydroxide ions (OH-) produced by the electrolysis of water remain in solution and react to form sodium hydroxide (NaOH). Therefore, the correct option is "sodium hydroxide, hydrogen, and chlorine" but it is not listed as an option. The closest option is "hydrogen and chlorine."

Equal amounts of marble chips are reacted separately with 100cm³ of hydrochloric acid of different concentrations. If all the marble chips reacted; which of the following remained the same in each case?

Detalles de la respuesta

The total mass of gas evolved during the reaction remained the same in each case. This is because equal amounts of marble chips were reacted separately with 100cm³ of hydrochloric acid of different concentrations. Since the same amount of marble chips was used in each reaction, the total amount of gas produced by the reaction will be equal, regardless of the concentration of the hydrochloric acid used. The other options, such as the average rate of evolution of gas, time taken for the reaction to reach completion, and initial reaction rates, may differ depending on the concentration of hydrochloric acid used.

Consider the following equation; Cr₂O_7(aq)^2- + 14H⁺_(aq) + 6e^- → 2Cr³⁺_(aq) + 7H₂O_(I). The oxidation number of chromium changes from

Detalles de la respuesta

In the given equation, the reactant Cr₂O₇^2- has chromium in the +6 oxidation state (since each oxygen atom has an oxidation number of -2, and there are seven of them, the total negative charge of the oxygen atoms is -14, and the total negative charge of the ion is -2, so the total positive charge of the chromium atoms must be +12 to balance it out). In the product side, the product Cr³⁺ has a +3 oxidation state. This means that the oxidation number of chromium has changed from +6 to +3, a decrease of 3. Therefore, the answer is - 2 to + 6, since chromium goes from +6 to +3.

A substance is said to be hygroscopic if it absorbs

Detalles de la respuesta

A substance is said to be hygroscopic if it absorbs moisture from the atmosphere without dissolving. This means that the substance has the ability to attract and hold water molecules from the air around it, but it does not dissolve in the water to form a solution. Hygroscopic substances are commonly used as desiccants to control the moisture content of products such as foods, medicines, and electronic components. Examples of hygroscopic substances include salt, sugar, silica gel, and calcium chloride.

(a)(i) State three methods of preparing salts, giving one example in each case of a salt so prepared.

(ii) What type of salt is each of the following? NaH\(_2\)PO\(_4\); (CH\(_3\)COO)\(_2\)Pb; KAI(SO\(_4\))\(_2\). 12H\(_2\)O.

(b)(i) Write an equation for the reaction between dilute HCI and a solution of AgNO\(_3\).

(ii) Explain why NaNO\(_3\) is preferred to AgNO\(_3\) in the preparation of oxygen by thermal decomposition of trioxonitrate (V) salts.

(iii) When silver wire was dipped into an aqueous solution of CuSO\(_4\), the wire remained intact but when the wire was replaced with zinc rod, the rod decreased in size. Give an explanation for this observation.

(c) When a sample of a crystalline salt X was exposed to air, there was a loss in mass.

(i) What phenomenon was exhibited by X?

(ii) Suggest two substances which X could be.

(iii) On heating 5.00 g of a fresh sample of X to constant mass, 1.80g was lost in the form of water vapour. Calculate the number of molecules of water of crystallization in one molecule of X. [H = 1.00; O = 16.00; Anhydrous form of X = 160 g mol\(^{-1}\)

Detalles de la respuesta

None

a)(i) Give two uses of ammonia.

(ii) Name the process by which ammoniacal liquor can be obtained from coal and list two other products of the reaction

(iii) What type of reaction is involved in the conversion of ammoniacal liquor to (NH\(_4\))\(_2\)SO\(_4\) by dilute H\(_2\)SO\(_4\)?

(iv) Sketch and label an energy profile diagram to show the effect of presence of Pt/Rh on the reaction represented by the following equation: 4NH\(_3\) + 5O\(_2\)  \(\to\) 6H\(_2\)O + 4NO; \(\Delta\)H = —907 kJmol\(^1\)

(b) Rock salt is an impure form of sodium chloride.

(i) Outline a suitable procedure for preparing a pure sample of sodium chloride from rock salt.

(ii) State two methods that can be used to prepare chlorine from rock salt. Write an appropriate equation in each case.

(c) Lead pigments were used in a water colour painting which turned black after prolonged exposure to an air pollutant. The original colour was restored by using H\(_2\)O\(_2\) which converted the black substance to a simple, white lead (II) salt.

(i) Which pollutant turned the painting black?

(ii) Write the formula of the black substance

(iii) What is the white salt?

(iv) State the role of H\(_2\)O in the restoration process.

Detalles de la respuesta

None

(a) Giving different examples, mention one metal in each case which produces hydrogen on reacting with

(i) dilute mineral acid

(ii) cold water;

(iii) steam;

(iv) hot, concentrated alkali.

(b) In an experiment, excess 0.50 mol dm\(^{-3}\) HCI was added to 1Og of granulated zinc in a beaker. Other conditions remaining constant, state how the reaction rate would be affected in each case, if the experiment was repeated using:

(i) 1.0 mol dm\(^{-3}\) HCI;

(ii) 8.0g of granulated zinc;

(iii) 10g of zinc dust;

(iv) a higher volume of 0.50 mol dm HCI;

(v) a reaction vessel dipped in crushed ice;

(vi) equal volumes of water and 0.50 mol dm\(^3\) HCI.

(c) Aluminium is extracted from its ore by electrolysis.

(i) Name the ore from which the metal is extracted.

(ii) State the role of molten cryolite in the extraction.

(iii) Describe in outline how the ore is purified before electrolysis

(iv) Calculate the current in amperes required to produce 18.0g of aluminium in 1.50 hours. [Al = 27.0; F = 96500C]

(d) Give the reason why

(i) aluminium, which is a reactive metal, is resistant to corrosion.

(ii) metals are generally good reducing agents.

Detalles de la respuesta

None

TEST OF PRACTICAL KNOWLEDGE QUESTION

All your burette readings (initial and final), as well as the size of your pipette, must be recorded but no account or expeimental procedure is required. All calculations must be done in your answer book.

A is 0.125 mol dm\(^3\) H\(_2\)SO\(_4\). B is a solution containing X g dm\(^{-3}\) of NaOH.

(a) Put A into the burette and titrate it against 20.0 cm\(^3\) or 25.0 cm\(^3\) portions of B using methyl orange as indicator. Record the volume of your pipette. Tabulate your burette readings and calculate the average volume of A used. The equation for the reaction involved in the titration is H\(_2\)SO\(_4\) +2NaOH\(_{(aq)}\) \(\to\) Na\(_2\)SO\(_4\) + 2H\(_2\)O\(_{(l)}\)

(b) From your results and the information provided above, calculate the;

(i) amount of H\(_2\)SO\(_4\) in the average volume of A used

(ii) Concentration of B in mol dm\(^{-3}\)

(iii) value of X.

[H = 1: O = 16; Na = 23]

(c) Describe briefly a suitable laboratory procedure for obtaining pure water from the titration mixture. (No diagram is required)

Detalles de la respuesta

None

(a) Explain in terms of the kinetic theory why a tyre should not be overinflated.

(b)The following results were obtained at room temperature in an experiment to verify one of the gas laws using a glass syringe:

(i) Plot a graph of P against \(\frac{1}{v}\), using 1 cm to represent 0.01 atm on the vertical axis and 1cm to represent 0.02 unit on the horizontal axis.

(ii) Which of the gas laws is in agreement with the results?

(c) The flow chart below represents the stages involved in the manufacture of H\(_2\)SO\(_4\).

                                 +x          +Conc. H\(_2\)SO\(_4\)                +H\(_2\)O

S + O\(_2\) \(\to\) SO\(_2\)      \(\to\)     SO\(_3\)         \(\to\)          Y      \(\to\)    Conc H\(_2\)SO\(_4\)

    stage I           stage II                 stage III                 stage IV    

(i) Name the process represented by the chart.

(ii) Identify reactant X and product Y.

(iii) What are the operating temperature and pressure at stage II?

(iv) Mention the stage which requires a catalyst and state the catalyst used.

(v) Give the reason why the SO\(_3\) produced in stage II is not dissolved directly in water to form the acid

(d) When K\(_4\)Cr\(_2\)C\(_7\) dissolves in water, the following equilibrium is established:

Cr\(_2\)O\(^{2-}_{7(aq)}\) + H\(_2\)O\(_{(l)}\)   \(\to\)  2CrO\(^{2-}_{4(aq)}\)  + 2H\(_{aq}\)

(i) State the colour observed on adding a few drops of dilute H\(_2\)SO\(_4\) to the system.

(ii) Explain your answer in (d)(1).

(iii) What principle is applicable to this explanation?

Detalles de la respuesta

None

TEST OF PRACTICAL KNOWLEDGE QUESTION

(a) Suggest how the following liquid reagents can be suitably stored in the laboratory   

(i) X which fumes in moist air; 

(ii) Y which is slowly decomposed by sunlight in ordinary reagent bottles.

(b) State what is observed when aqueous ammonia is added to:

(i) litmus paper;

(ii) Pb(NO\(_3\))\(_2\) solution in drops until in excess

(iii) freshly precipitated AgCI in excess.

(c) A salt sample was suspected to be either Na\(_2\)CO\(_2\) or NaHCO\(_3\). A student who was required to identify it tested a portion for solubility in water and then for effect on litmus paper.

(i) What was observed in each case?

(ii) Give the reason why the student's procedure was unsuitable

(iii) Describe briefly how you would have identified the salt.

Detalles de la respuesta

None