Question

4. (a)(i) Define the term hygroscopic. (ii) Give two differences between a physical change and a chemical change. (iii) Using the kinetic theory of gases, explain briefly the Charles' law.
(b)(i) Arrange the following compounds in order of increasing boiling points: CS2; CO2; NaH. Give reasons for your answer.
(ii) Write a balanced chemical equation to illustrate the reaction of chlorine gas with cold dilute sodium hydroxide.
(c) In a certain reaction, 15.0g of impure magnesium sample reacted with excess hydrochloric acid liberating 8.6dm³ of hydrogen gas at s.t.p.
(i) Write a balanced equation for the reaction.
(ii) Calculate the: I. mass of pure magnesium in the sample; II. percentage purity of the magnesium sample; III. number of Cl⁻ ions produced in the reaction. [Mg=24.0; volume at s.t.p.=22.4dm³; Avagadro's constant=6.02×10²³mol⁻¹]

Accepted Answer

(a)(i) A substance is said to be hygroscopic if when exposed to the atmosphere, it can absorb moisture to become wet or sticky. Examples of hygroscopic substances are CaO, CI0, H2SO4 etc.
(ii) A physical change is easily reversible while a chemical change is not easily reversible. A physical change does not produce any new substance while a chemical change produces new substance(s).
(iii) At a constant pressure, when the temperature of a gas is increased, the molecules of the gas gain more kinetic energy. They therefore move faster with the distance between the molecules increasing, leading to an apparent increase in volume.

(b)(i) Increasing boiling point: CO2 < CS2 < NaH. Reason: NaH is an ionic substance. Thus, it has the highest boiling point. CS2 has stronger Van der Waals forces than CO2. Hence, it has a higher boiling point than CO2.
(ii) Cl2(g) + 2NaOH(aq) → NaCl(aq) + NaOCl(aq) + H2O(l). (Note: if the alkali is hot and concentrated, the equation will instead be 3Cl2+6NaOH(aq) → 5NaCl+NaClO3+3H2O.)

(c)(i) Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g).
(ii) I. Mole of H2 = Volume of hydrogen at s.t.p./GMV = (8.6/22.4) mol = 0.383 mol. Since both magnesium and hydrogen are equal in mole (from the balanced equation), mole of magnesium used = 0.3839 mol. Molar mass of magnesium = 24g/mol. Mass of pure magnesium = (0.3839×24)g = 9.2136 g.
II. % purity of the magnesium sample = (Mass of pure sample/Mass of impure sample)×100% = (9.2136g/15g)×100% = 61.42%.
III. To calculate number of chloride ions produced: since both MgCl2 and H2 are equal in mole (from the balanced equation), mole of MgCl2 = 0.3839 mol. 1 mole of MgCl2 produces 2 moles of Cl⁻. Thus, mole of Cl⁻ = (0.3839×2) mol = 0.7678 mol. Number of chloride ions = mole × Avogadro's constant = 0.7678×6.02×10²³ = 4.622×10²³ ions.

WAEC Chemistry 2013 Theory — Question 4

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