WAEC Chemistry 2010 Theory Past Questions

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Alpha particle, beta particle and gamma ray. Nature/properties of alpha particle: it is represented by ⁴₂α or ⁴₂He; it has a low penetration power; it has a high ionizing power. Properties of beta particle: it is represented by ⁰₋₁β or ⁰₋₁e; it has a moderate penetration power; it has a moderate ionizing power; it is negatively charged, hence it is deflected towards the positive plate in an electric field. Properties of gamma ray: it is represented by ⁰₀γ; it has a very high penetration power; it has a low ionizing power; it is electrically neutral.

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Chemical reactions involve only bonding electrons while nuclear reactions involve protons and neutrons. The identity of each element is retained during a chemical reaction, while nuclear reactions occur with a change of identity. A small amount of energy is involved in a chemical reaction while nuclear reactions involve a huge amount of energy.

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I. ²¹²₈₄PO → ²⁰⁸₈₂Pb + X, where X = Alpha particle (⁴₂He). II. ¹³⁷₅₅Cs → ¹³⁷₅₆Ba + Y, where Y = Beta particle (⁰₋₁e).

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(i) The atomic number of X is 17. (ii) Group (VII). (iii) Properties: they form monovalent negative ions; they are highly electronegative; they are very good oxidizing agents. (iv) X is chlorine. (v) 3Cl2(g)+6NaOH(aq)→5NaCl(aq)+NaClO3(aq)+3H2O(l).

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I. Graphite is used as a lubricant because it consists of planar layers which can slide over one another. II. Diamond is very hard, so it is used for making cutting tools.

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NH3(aq)+HCl(g)→NH4Cl(aq), or NH4OH+HCl→NH4Cl+H2O.

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Neutralization reaction.

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A saturated solution is a solution that contains the maximum amount of solute it can hold at a particular temperature, in the presence of undissolved solute particles.

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Mass = 67.0g. Molar mass of KNO3 = 101 gmol⁻¹. Mole of KNO3 = 67.0g/101gmol⁻¹ = 0.6634 mol. Volume = 250cm³ = 0.25dm³. Concentration = 0.6634mol/0.25dm³ = 2.6536 moldm⁻³. Since the solubility of KNO3 at 20°C (3 moldm⁻³) is greater than the concentration of the solution formed (2.6536 moldm⁻³), the solution is unsaturated.

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In covalent bonding, all the participants donate electrons to be shared, while in dative bonding, only one of the participants donates both the electrons to be shared.

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Sugar does not conduct electricity because it is composed of molecules rather than ions. Common salt, although it contains ions, does not conduct electricity in the solid form because its ions are not mobile in the solid form.

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I. The intermolecular force present in hydrogen fluoride is hydrogen bond. II. The intermolecular forces present in Argon are Van der Waal's forces.

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The hydrogen bond is stronger in O—H···N. This is because oxygen is smaller and more electronegative than sulphur, so O-H forms a stronger hydrogen bond.

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Dalton's law of partial pressure states that if there is a mixture of gases which do not react chemically, the total pressure exerted by the mixture is the sum of the pressure exerted by the individual gases.

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V1=200cm³, T1=18°C=291K. P1=700mmHg-15mmHg=685mmHg. V2=? T2=273K, P2=760mmHg. Using P1V1/T1 = P2V2/T2: V2 = P1V1T2/(P2T1) = (685x200x273)/(760x291) = 169.11 cm³.

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A base is a substance which produces hydroxyl (OH⁻) ions as the only negative ion in its solution.

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Examples of a base include NaOH, KOH, Ca(OH)2 (any one).

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C12H22O11(aq) (sucrose) => Non-electrolyte. NH3(aq) => Weak electrolyte. NaOH(aq) => Strong electrolyte.

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CH3COOH+KOH→CH3COOK+H2O.

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Using the formula CaVa/CbVb = na/nb: Va = (CbVbna)/(Canb) = (0.3x20x1)/(0.5x1) = 12cm³.

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I. NaOCl => Sodium oxochlorate(I). II. Mg(HCO3)2 => Magnesium hydrogentrioxocarbonate (IV).

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A standard solution is a solution with a known concentration.

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Na2CO3 is suitable for making a standard solution because it does not absorb water from the atmosphere, i.e. it is not deliquescent or hygroscopic.

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Mole of HCl = (50/1000)dm³ x 2.20moldm⁻³ = 0.11 mol. Since mole of Fe is half that of HCl (from the balanced equation), mole of iron = 0.5 x 0.11 = 0.055 mol. Molar mass = mass of Fe/mole of Fe = 3.08g/0.055mol = 56 g/mol; thus, the relative atomic mass of iron is 56.

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Soda lime does not attack glass apparatus (soda lime is not deliquescent, unlike caustic soda which is corrosive to glass and hygroscopic).

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Methane is a gas at room temperature; it is less dense than air.

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C: 82.76/12 = 6.9967. H: 17.24/1 = 17.24. Dividing by the smaller (6.9967): C=1, H=2.4643≈2.5. Multiplying through by 2: C2H5. Empirical formula = C2H5. RMM = 2x29 = 58. Molecular formula (C2H5)n = 58, giving (2x25+5)n = 58, 29n=58, n=2. Molecular formula = C4H10.

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Isomerism is the existence of two or more compounds with the same molecular formula but different arrangement of atoms.

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(a) H-C(H)(H)-O-C(H)(H)(H) i.e. methoxymethane, CH3-O-CH3. (b) H-C(H)(H)-C(H)(H)-O-H i.e. ethanol, C2H5OH.

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'a' is methoxymethane while 'b' is ethanol.

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'a' is an alkoxyalkane while 'b' is an alkanol.

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(i) The types of elements present. (ii) The amount of a given chemical entity. (iii) The types of bonds present.

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The two gases should be passed separately into bromine water. The one that decolorizes bromine water is propene, while the one that shows no reaction is propane.

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Not applicable to Nigerian candidates - omitted from this paper.

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(i) Solid - coke. (ii) Liquid - ammoniacal liquor. (iii) Gas - coal gas.

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Hydrogen gas.

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It is because synthetic gas does not contain sulphur or sulphur compounds.

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C(s)+O2(g)→CO2(g).

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I. Physical properties: Carbon (IV) oxide is gaseous at room temperature; it is denser than air. II. Chemical properties: it reacts with red hot carbon to give carbon monoxide, CO: CO2(g)+C(s)→2CO(g); it reacts with lime water to form calcium trioxocarbonate (IV) or calcium hydrogentrioxocarbonate (IV) if in excess: CO2(g)+Ca(OH)2(aq)→CaCO3(s)+H2O(l); 2CO2(g)+Ca(OH)2(aq)→Ca(HCO3)2(aq).

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I. Crystalline allotropes of carbon: diamond and graphite. II. Amorphous allotropes of carbon: coal, coke, charcoal, soot.

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S(s)+O2(g)→SO2(g); 2SO2(g)+O2(g)⇌2SO3(g); SO3(g)+H2SO4(aq)→H2S2O7(l) [oleum]; H2S2O7(l)+H2O(l)→2H2SO4(aq). (ii) In these equations, H2SO4 (as oleum precursor and product) acts as a dehydrating agent and as a precipitating agent depending on the reaction context.

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Water is regarded as the universal solvent because it can dissolve many substances.

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The suspected substance should be brought in contact with cobalt (II) chloride paper. If the colour changes from blue to pink (or anhydrous CuSO4 turns from white to blue), the suspected substance contains water.

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Oxidation half equation: 4OH⁻→2H2O+O2+4e⁻. Reduction half equation: 2H⁺(aq)+2e⁻→H2(g). Overall equation: 4OH⁻(aq)+4H⁺(aq)→2H2O(l)+H2(g)+O2(g).

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I = 1.25A, t = 40 mins = (40x60)s = 2400s. Q = It = (1.25x2400)C = 3000C. 4OH⁻→2H2O+O2+4e⁻ requires 4 moles of electrons (4Fx96500C) to produce 1 mole of O2 (22.4 dm³ at s.t.p). Hence, 3000C will produce (22.4dm³x3000C)/(4x96500C) = 0.174 dm³ = 174 cm³.

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Mole of Au = 1.25/197 = 0.006345 mol. Mole of Cl2 = 1.744/71 = 0.0256 mol. From the balanced equation, 2 moles of Au require 3 moles of Cl2 (i.e. 2/2 x 0.006345 mol of Au require 3/2 x0.006345 mol of Cl2) = 0.009518 mol of Cl2. Since the amount of Cl2 given (0.02456 mol) is greater than the amount required (0.009518 mol), Cl2 is the excess reagent. Excess amount = (0.02456-0.009518) mol of Cl2 = 0.01594 mol of Cl2.