WAEC SSCE - Chemistry - 2010

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Maelezo ya Majibu

The quantity of electricity required to discharge 1 mole of univalent ion is known as the Faraday constant, which is equal to the charge of 1 mole of electrons and has a value of approximately 96,500 coulombs (C). This means that the correct answer is, 96,500 C. Therefore, to discharge 1 mole of univalent ions, a charge of 96,500 C is required.

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The determination of heat of combustion involves measuring the amount of heat energy released when a substance is burned completely in oxygen. To measure this energy, a device called a bomb calorimeter is used. The substance is placed in a sealed container (the "bomb"), and then ignited by an electric spark. As the substance burns, the heat produced raises the temperature of the surrounding water, which is used to calculate the heat of combustion. Therefore, the correct answer is "a bomb calorimeter."

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Out of the given options, CaO is a basic oxide. Basic oxides are also called alkaline oxides or simply bases, which have a pH value greater than 7. When they react with water, they produce metal hydroxides, which are also bases. CaO, or calcium oxide, is a metal oxide that reacts with water to produce Ca(OH)₂, which is a strong base. In contrast, NO₂ and SO₂ are acidic oxides, and Al₂O₃ is an amphoteric oxide, which means it can behave as either an acid or a base depending on the conditions.

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A molecule is linear if all of its atoms lie in a straight line. Of the given molecules, only CO₂ has a linear shape because it has a central carbon atom double-bonded to two oxygen atoms, which are both linearly arranged on opposite sides of the carbon atom, forming a straight line. CH₄, H₂S, and NH₃ have a tetrahedral, bent, and pyramidal shape respectively, meaning they are not linear.

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In the given redox reaction, the oxidation state of manganese (Mn) changes from +7 in MnO₄^- to +2 in Mn²⁺. Therefore, the change in oxidation state of manganese is from +7 to +2. The oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were completely ionic. When an element undergoes oxidation, its oxidation state increases, and when it undergoes reduction, its oxidation state decreases. In this reaction, manganese is undergoing reduction since its oxidation state is decreasing from +7 to +2, which means it is gaining electrons.

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The energy change which accompanies the addition of an electron to a gaseous atom is called "electron affinity". In simple terms, it is the energy released or absorbed when a neutral gaseous atom gains an electron to form a negatively charged ion. This energy change is often expressed in units of kilojoules per mole (kJ/mol). A positive electron affinity value indicates that energy is absorbed when an electron is added, whereas a negative value indicates that energy is released.

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Out of the given options, only N₂ has a triple bond in its structure. A triple bond is a type of covalent bond in which three pairs of electrons are shared between two atoms. In the case of N₂, the two nitrogen atoms share three pairs of electrons, resulting in a triple bond between them. The other options do not have a triple bond in their structure. CH₄ has four single covalent bonds, NH₃ has three single covalent bonds, and O₂ has a double bond between the two oxygen atoms.

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The stoichiometric ratio of acid to base refers to the ratio of the coefficients of the acid and base in a balanced chemical equation. In the given equation, the balanced equation is: Na2CO3 + 2HNO3 → 2NaNO3 + H2O + CO2 The coefficients of HNO3 and Na2CO3 are 2 and 1 respectively. Therefore, the stoichiometric ratio of acid to base is 2:1. This means that for every 2 moles of HNO3 used in the reaction, 1 mole of Na2CO3 reacts.

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A measuring cylinder can be used to measure accurately a specific volume of a liquid. Measuring cylinders have graduated scales on the side, which allow for precise measurement of the volume of a liquid. They come in different sizes and are often used in experiments where the volume of a liquid needs to be measured with a high degree of accuracy. Beakers and conical flasks, on the other hand, do not have precise measurement markings and are therefore not ideal for accurately measuring specific volumes of liquids. Pipettes, however, can also be used to accurately measure a specific volume of a liquid, and are often used when very precise measurements are required.

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Among the given options, AgCl is the only chloride that is insoluble in water. This is because AgCl has a very low solubility in water, which means that only a small amount of it can dissolve in water. In contrast, KCl, NH₄Cl, and ZnCl₂ are all soluble in water, meaning they can easily dissolve in water to form a homogeneous solution. The solubility of a salt in water depends on various factors such as the nature of the ions, temperature, and pressure.

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The main function of limestone in the blast furnace is to remove impurities from the iron ore being smelted. Limestone, which is mostly calcium carbonate, reacts with the impurities in the iron ore to form slag, which is then easily separated from the molten iron. The chemical reaction is as follows: CaCO₃(s) + SiO₂(l) → CaSiO₃(l) + CO₂(g) Thus, limestone acts as a flux, a substance that can lower the melting point of the iron ore and helps to remove impurities from the ore. It does not act as a reducing agent, catalyst, or supply carbon (IV) oxide.

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The minimum amount of energy required for effective collisions between reacting particles is known as "activation energy." In simple terms, it is the energy required to start a chemical reaction by breaking the bonds of the reactants. It is the energy barrier that needs to be overcome before a chemical reaction can occur. Once this energy barrier is overcome, the reaction proceeds on its own, releasing energy in the form of heat or light. The activation energy is dependent on the nature of the reactants and the reaction conditions such as temperature and pressure.

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To determine the empirical formula of the compound, we need to find the simplest whole number ratio of the atoms present in the compound. Assume we have 100g of the compound. This means that we have: - 40.0g of Carbon - 6.7g of Hydrogen - 53.3g of Oxygen Next, we need to convert the mass of each element to moles. We can do this by dividing the mass of each element by its atomic weight: - Moles of Carbon = 40.0g ÷ 12.0g/mol = 3.33 moles - Moles of Hydrogen = 6.7g ÷ 1.0g/mol = 6.7 moles - Moles of Oxygen = 53.3g ÷ 16.0g/mol = 3.33 moles We can then divide each of the mole values by the smallest mole value to get the simplest whole number ratio: - Carbon: 3.33 ÷ 3.33 = 1 - Hydrogen: 6.7 ÷ 3.33 = 2.01 (approx. 2) - Oxygen: 3.33 ÷ 3.33 = 1 Therefore, the empirical formula of the compound is CH₂O. The correct answer is (c) CH₂O.

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Iron is a d-block element. The d-block elements are also known as the transition metals, which are located in the middle of the periodic table between the s-block elements and the p-block elements. Iron (Fe) is a transition metal with an atomic number of 26, and it is located in the d-block. The d-block elements have partially filled d-orbitals, which give them unique properties such as variable oxidation states, colorful complexes, and catalytic activity.

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The correct answer is "ethanol and carbon (IV) oxide". Fermentation is a metabolic process that converts sugar (glucose) into alcohol (ethanol) and carbon dioxide through the action of enzymes in the absence of oxygen. The reaction is catalyzed by yeast or bacteria, which break down the glucose molecule into two molecules of pyruvate in a process called glycolysis. Pyruvate then undergoes further chemical reactions that produce ethanol and carbon dioxide as waste products. The balanced chemical equation for the fermentation of glucose is: C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂ So, the products of fermentation of sugar are ethanol and carbon dioxide.

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An ion is an atom or molecule that has lost or gained one or more electrons and has a net electric charge. In this question, the element M has 13 protons and 10 electrons, which means it has a net charge of +3 to balance out the charges of the protons and electrons. Therefore, the correct answer is ₁₃M ³⁺. The other options are incorrect because they either have the wrong number of protons or the wrong number of electrons to match the given element.

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A solid substance with high melting and boiling points is likely to be an electrovalent compound. Electrovalent compounds (also known as ionic compounds) are formed by the transfer of electrons between a metal and a non-metal, resulting in the formation of a lattice structure held together by strong electrostatic forces between positively and negatively charged ions. These strong intermolecular forces make it difficult to break the lattice structure, leading to high melting and boiling points. Covalent compounds, on the other hand, typically have weaker intermolecular forces, resulting in lower melting and boiling points. Dative covalent compounds are a type of covalent compound where both electrons in a shared pair are donated by one atom, and non-metals are a broad category of elements that can form both covalent and electrovalent compounds.

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The volume occupied by 1 mole of any ideal gas at standard temperature and pressure (STP) is 22.4 dm³ or 22.4 L. STP is defined as a temperature of 273 K (0 °C) and a pressure of 1 atm (101.325 kPa). Therefore, the volume occupied by 2 moles of ammonia at STP would be: V = 2 × 22.4 dm³ = 44.8 dm³ So the correct option is (a) 44.8 dm^-3.

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Group III elements form oxides in which the oxidation state of the metal ion is +3. The chemical formula for these oxides is M₂O₃, where M represents the Group III element. Therefore, the oxide of a Group III element X would be represented as X₂O₃. Option (c) is the correct answer.

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Mercury is the only metal among the given options that exists as a liquid at ordinary temperature. Copper, gold, and silver are all solid at room temperature and require high temperatures to melt and become liquid. Mercury, on the other hand, has a melting point of -38.83°C and a boiling point of 356.73°C, making it liquid at room temperature. Therefore, the correct answer is "Mercury".

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To calculate the concentration of the sodium trioxocarbonate (IV) solution, we need to use the formula: Concentration (in mol dm^-3) = (mass ÷ molar mass) ÷ volume Firstly, we need to calculate the number of moles of Na₂CO₃ in the solution: mass of Na₂CO₃ = 10.6 g molar mass of Na₂CO₃ = 106.0 g mol^-1 Number of moles of Na₂CO₃ = mass/molar mass = 10.6/106.0 = 0.1 mol Next, we can use the formula above to calculate the concentration: Concentration (in mol dm^-3) = (mass ÷ molar mass) ÷ volume Volume = 250 cm³ = 0.25 dm³ Concentration (in mol dm^-3) = (0.1 mol ÷ 0.25 dm³) = 0.4 mol dm^-3 Therefore, the concentration of the sodium trioxocarbonate (IV) solution is 0.4 mol dm^-3. The correct option is (A) 0.4 mol dm^-3.

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The correct equation that represents beta decay is: \(\frac{8238}{92}\)U → \(\frac{239}{93}\)Np + \(\frac{0}{-1}\)e In beta decay, a neutron in the nucleus of an atom breaks down into a proton and an electron. The electron is then emitted from the nucleus, and the proton remains in the nucleus. This results in the atomic number of the nucleus increasing by one, but the mass number remaining the same. The equation above represents the beta decay of Uranium-238 into Neptunium-239 by the emission of an electron (0/-1 e).

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Chlorine and iodine are both nonmetals that exist as diatomic molecules, which means they are made up of two atoms of the same element. At room temperature (around 25°C or 77°F), chlorine is a yellow-green gas, while iodine is a shiny black or purple solid. Therefore, the correct statement is: "Chlorine is a gas and iodine is a solid."

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The substance that is a detergent and meets the criteria given is "lather readily with water." This is because detergents are a type of surfactant that lowers the surface tension of water, allowing it to penetrate and lift away dirt and oil from surfaces. One of the characteristics of detergents is that they produce foam or lather when mixed with water, which helps to distribute the detergent evenly and aid in cleaning. Therefore, the correct option is "lather readily with water."

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At standard temperature and pressure (STP), which is defined as a temperature of 273 K (0°C) and a pressure of 1 atm (101.3 kPa), all gases occupy the same volume of 22.4 L per mole. Therefore, the number of atoms present in a gas is directly proportional to the number of moles of that gas. Thus, the gas with the highest number of atoms at STP would be the one with the highest number of moles. Using Avogadro's law, 1 mole of any gas at STP contains approximately 6.02 x 10^23 atoms or molecules. - 6 moles of neon would contain 6 x (6.02 x 10^23) atoms = 3.61 x 10^24 atoms - 3 moles of oxygen would contain 3 x (6.02 x 10^23) atoms = 1.81 x 10^24 atoms - 2 moles of chlorine would contain 2 x (6.02 x 10^23) atoms = 1.20 x 10^24 atoms - 1 mole of ethane (C2H6) would contain 1 x (2 x 6.02 x 10^23) atoms = 1.20 x 10^24 atoms Therefore, the gas with the highest number of atoms at STP is neon with 6 moles, which contains 3.61 x 10^24 atoms.

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A normal salt is formed by the complete replacement of the hydrogen ions of an acid by a metal ion or ammonium ion. Ammonium salts are also normal salts. Out of the given options, all are salts except for Mg(OH)NO₃, which is a complex salt. This is because Mg(OH)₂ is a weak base and reacts partially with nitric acid to form a complex salt, which contains the anions of both Mg(OH)₂ and HNO₃. Thus, the correct answer is Mg(OH)NO₃.

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None

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(a) Saturated solution

• (i) A saturated solution is one that contains the maximum amount of dissolved solute that it can hold at a given temperature, in the presence of undissolved solute (any further solute added will not dissolve).
• (ii) Solubility of KNO\(_3\) at 20 degrees Celsius \(= 3.00\ \text{mol dm}^{-3}\). Amount added: molar mass of KNO\(_3 = 101.0\), so \[n = \frac{67.0}{101.0} = 0.663\ \text{mol}\ \text{in}\ 250\ \text{cm}^3\ (0.250\ \text{dm}^3).\] Concentration formed \[= \frac{0.663}{0.250} = 2.65\ \text{mol dm}^{-3}.\] Since \(2.65\ \text{mol dm}^{-3} < 3.00\ \text{mol dm}^{-3}\) (the solubility), the solution is unsaturated, i.e. it is not saturated at 20 degrees Celsius.

(b) Bonding

• (i) In an ordinary covalent bond each of the two bonded atoms contributes one electron to the shared pair, whereas in a dative (co-ordinate) bond both electrons of the shared pair are supplied by only one of the atoms.
• (ii) Sugar and common salt do not conduct in the solid state because sugar is a covalent (molecular) solid with no ions or free electrons, and although common salt is ionic, in the solid its ions are held rigidly in the lattice and are not free to move; conduction needs mobile charge carriers.
• (iii) I. Hydrogen fluoride: hydrogen bonding (plus weaker dipole-dipole/van der Waals). II. Argon: weak van der Waals (dispersion) forces only.
• (iv) The hydrogen bond is stronger in O-H----N. Reason: oxygen is more electronegative than sulphur, so the O-H bond is more polar and the hydrogen carries a greater partial positive charge, giving a stronger attraction to the nitrogen lone pair.

(c) Gas laws

• (i) Dalton's law of partial pressures: in a mixture of gases that do not react, the total pressure is equal to the sum of the partial pressures of the individual gases, each partial pressure being the pressure the gas would exert if it alone occupied the whole volume at the same temperature.
• (ii) Pressure of dry CO\(_2\) \(= 700 - 15 = 685\ \text{mmHg}\); \(V_1 = 200\ \text{cm}^3\), \(T_1 = 18 + 273 = 291\ \text{K}\). At s.t.p. \(P_2 = 760\ \text{mmHg}\), \(T_2 = 273\ \text{K}\). Using \(\dfrac{P_1V_1}{T_1} = \dfrac{P_2V_2}{T_2}\): \[V_2 = \frac{P_1V_1T_2}{T_1P_2} = \frac{685 \times 200 \times 273}{291 \times 760} = 169.1\ \text{cm}^3.\] The volume of the dry gas at s.t.p. is about 169 cm\(^3\).

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(a) Water

• (i) Water is called a universal solvent because it dissolves a very large number of substances (many ionic and polar solids, liquids and gases), more than any other common solvent, owing to its polar nature.
• (ii) Chemical test for water: add the liquid to white anhydrous copper(II) sulphate, which turns blue; or to blue anhydrous cobalt(II) chloride paper, which turns pink. (To show it is pure water, it also boils at 100 degrees Celsius.)

(b) Electrolysis of dilute H\(_2\)SO\(_4\)

• (i) At the anode (oxidation), oxygen is liberated: \[2H_2O_{(l)} \to O_{2(g)} + 4H^{+}_{(aq)} + 4e^{-}\]
• (ii) Quantity of charge \(Q = It = 1.25 \times (40\times60) = 1.25 \times 2400 = 3000\ \text{C}\). Moles of electrons \(= \dfrac{3000}{96500} = 0.03109\) mol. From the equation, 4 moles of electrons give 1 mole of O\(_2\): \[n(O_2) = \frac{0.03109}{4} = 7.77\times10^{-3}\ \text{mol}.\] Volume at s.t.p. \(= 7.77\times10^{-3} \times 22.4 = 0.174\ \text{dm}^3\) (about 174 cm\(^3\)).

(c) Au and Cl\(_2\)

• (I) \[2Au + 3Cl_2 \to 2AuCl_3\]
• (ii) I. Moles of Au \(= \dfrac{1.250}{197.0} = 6.35\times10^{-3}\) mol; moles of Cl\(_2\) \(= \dfrac{1.744}{71.0} = 2.456\times10^{-2}\) mol. From the equation, 2 mol Au need 3 mol Cl\(_2\), so 6.35 x 10\(^{-3}\) mol Au need \(\dfrac{3}{2}\times6.35\times10^{-3} = 9.52\times10^{-3}\) mol Cl\(_2\). Since 2.456 x 10\(^{-2}\) mol Cl\(_2\) is available (more than 9.52 x 10\(^{-3}\)), chlorine is in excess (gold is the limiting reactant). II. Excess Cl\(_2\) \(= 2.456\times10^{-2} - 9.52\times10^{-3} = 1.50\times10^{-2}\) mol, i.e. mass \(= 1.50\times10^{-2} \times 71.0 = 1.07\ \text{g}\) of chlorine.

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(a)(i) Soda lime is used instead of caustic soda because:

1. Soda lime does not attack the glass apparatus on heating, whereas caustic soda attacks glass.
2. Soda lime is not deliquescent, whereas caustic soda absorbs moisture from the air and becomes difficult to handle.

(ii) Methane is:

1. a colourless, odourless gas at room temperature;
2. slightly soluble in water and less dense than air.

(iii)

Vapour density = 29.

Therefore, molar mass = \(2 \times 29 = 58\).

Element	Percentage composition	Relative atomic mass	Mole ratio
C	82.76	12	\(82.76/12 = 6.90\)
H	17.24	1	\(17.24/1 = 17.24\)

Dividing by the smaller value, 6.90:

\[\text{C : H} = 1 : 2.5 = 2 : 5\]

I. Empirical formula = \(\mathrm{C_2H_5}\).

Empirical formula mass = \((2 \times 12) + (5 \times 1) = 29\).

\[n=\frac{58}{29}=2\]

II. Molecular formula = \((\mathrm{C_2H_5})_2 = \boxed{\mathrm{C_4H_{10}}}\).

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(b)(i) Isomerism is the existence of two or more compounds having the same molecular formula but different structural formulae, or different arrangements of atoms.

(ii) and (iii) The two isomers of \(\mathrm{C_2H_6O}\) are shown below.

(iv) The isomers have different functional groups and belong to different homologous series. Ethanol is an alkanol containing the hydroxyl group, \(\mathrm{-OH}\), while methoxymethane is an ether containing the \(\mathrm{-O-}\) linkage.

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(c) Qualitative and quantitative analysis of an organic compound can be used to deduce:

1. the types of elements present in the compound;
2. the percentage composition, and hence the relative numbers of atoms, of the elements present;
3. the empirical formula and, where the molar mass is known, the molecular formula of the compound.

(d) Pass each gas through bromine water. Propene decolourises the reddish-brown bromine water because it is unsaturated, whereas propane produces no colour change.

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(a) Acids, bases and electrolytes

• (i) According to Arrhenius, a base is a substance that dissociates in water to produce hydroxide ions (OH\(^-\)) as the only negative ions.
• (ii) Example: sodium hydroxide, NaOH (or KOH).
• (iii) I. C\(_{12}\)H\(_{22}\)O\(_{11}\) (sugar): non-electrolyte. II. NH\(_3\): weak electrolyte. III. NaOH: strong electrolyte.
• (iv) \[CH_3COOH + KOH \to CH_3COOK + H_2O\]

(b) Volume of HCl to neutralize 20.00 cm\(^3\) of 0.300 mol dm\(^{-3}\) NaOH (reaction 1:1). Moles NaOH \(= \dfrac{0.300 \times 20.00}{1000} = 6.00\times10^{-3}\) mol. Moles HCl needed = same = \(6.00\times10^{-3}\) mol. \[V = \frac{6.00\times10^{-3}}{0.500} = 0.0120\ \text{dm}^3 = 12.0\ \text{cm}^3.\]

(c) IUPAC names

• NaOCl: sodium chlorate(I) (sodium oxochlorate(I)).
• Mg(HCO\(_3\))\(_2\): magnesium hydrogentrioxocarbonate(IV) (magnesium hydrogencarbonate).

(d) Standard solution and iron

• (i) A standard solution is a solution whose concentration is accurately known.
• (ii) Na\(_2\)CO\(_3\) is the suitable one. Reason: sodium carbonate can be obtained pure, is stable and does not absorb moisture or carbon dioxide from the air, so a known mass gives an accurately known concentration; NaOH is deliquescent and absorbs CO\(_2\), so it cannot be weighed out accurately as a primary standard.
• (iii) I. \[Fe + 2HCl \to FeCl_2 + H_2\uparrow\] II. Moles of HCl \(= \dfrac{2.20 \times 50.0}{1000} = 0.110\) mol. From the equation Fe : HCl = 1 : 2, so moles of Fe \(= \dfrac{0.110}{2} = 0.0550\) mol. \[\text{Relative atomic mass} = \frac{\text{mass}}{\text{moles}} = \frac{3.08}{0.0550} = 56.0.\] So the relative atomic mass of the metal is 56.

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(a) Radioactivity

• (i) Radiations emitted: alpha (\(\alpha\)) radiation, positively charged helium nuclei; beta (\(\beta\)) radiation, negatively charged fast-moving electrons; gamma (\(\gamma\)) radiation, high-energy electromagnetic waves with no charge and no mass.
• (ii) Two differences between chemical and nuclear reactions: a chemical reaction involves only the outer (valence) electrons and the atomic nucleus is unchanged, whereas a nuclear reaction involves the nucleus and the element is changed into a different element; the energy change in a nuclear reaction is enormously greater than in a chemical reaction. (Also: chemical reactions can be affected by temperature/pressure/catalyst; nuclear reactions cannot.)
• (iii) Balancing:
  • I. \(^{212}_{84}Po \to\ ^{208}_{82}Pb +\ ^{4}_{2}X\). Mass number: \(212-208 = 4\); atomic number: \(84-82 = 2\). So X is an alpha particle (\(^{4}_{2}He\)).
  • II. \(^{137}_{55}Cs \to\ ^{137}_{56}Ba +\ ^{0}_{-1}Y\). Mass number unchanged (137); atomic number rises by 1 (55 to 56). So Y is a beta particle (\(^{0}_{-1}e\), an electron).

(b) Element X: \(1s^{2}2s^{2}2p^{6}3s^{2}3p^{5}\)

• (i) Atomic number: total electrons \(= 2+2+6+2+5 = 17\).
• (ii) Group: 7 valence electrons, so Group VII (17), the halogens.
• (iii) Two properties of the group: they are reactive non-metals existing as diatomic molecules; they are strong oxidizing agents whose reactivity decreases down the group; they form ions of charge \(-1\) (and coloured, with characteristic states, e.g. Cl\(_2\) is a greenish-yellow gas).
• (iv) Identity: X is chlorine.
• (v) Reaction with hot concentrated NaOH: \[3Cl_2 + 6NaOH \to 5NaCl + NaClO_3 + 3H_2O\]

(c) Carbon and neutralization

• (i) I. Graphite as a lubricant: graphite has a layered structure in which the flat layers are held together by weak forces, so the layers slide easily over one another, giving a slippery, lubricating action. II. Diamond as a cutting tool: in diamond every carbon atom is covalently bonded to four others in a rigid three-dimensional giant structure, making it extremely hard, so it can cut and grind other materials.
• (ii) \[HCl_{(aq)} + NH_3{(aq)} \to NH_4Cl_{(aq)}\]
• (iii) Type of reaction: neutralization (an acid-base reaction; also a combination reaction).