Question

3. (a) A compound contains 52.2% C, 13.1% H and Oxygen only. The vapour density of the compound is 23.
 (i) Determine its empirical formula;
 (ii) Determine its molecular formula;
 (iii) The compound reacts with sodium metal to produce hydrogen gas, and when warmed with acidified KMnO4(aq) gives a solution which turns from purple to colourless. It also forms a sweet-smelling liquid when heated with ethanoic acid in the presence of concentrated H2SO4.
  I. name the functional group present in the compound;
  II. draw the structural formula of the compound. [H=1.0, C=12.0, O=16.0]
(c) (i) Explain briefly why a piece of aluminium does not react with water.
 (ii) How can pure aluminium chloride crystals be obtained from an aluminium chloride solution?
(d) Describe briefly how water can be separated from aqueous CuSO4.

Accepted Answer

(a)(i) %O = 100 − (52.2 + 13.1) = 34.7

Element: C, H, O
% composition: 52.2, 13.1, 34.7
÷ Molar mass: 52.2/12 = 4.35; 13.1/1 = 13.1; 34.7/16 = 2.168
÷ smallest (2.168): C = 2.00, H = 6.04, O = 1.00
Empirical formula = C2H6O

(ii) 2 × vapour density = molecular mass; 2×23 = 46. (C2H6O)n = 46; [(12×2)+(1×6)+16]n = 46; 46n = 46; n = 1. ∴ Molecular formula = C2H6O

(iii) I. Hydroxyl (–OH) functional group — the compound is ethanol.
II. Structural formula: H–C(H)(H)–C(H)(H)–O–H (i.e. CH3–CH2–OH), each carbon bonded to the appropriate hydrogens and the terminal carbon bonded to the hydroxyl group.

(c)(i) When aluminium is exposed, the reaction with water starts but the metal quickly produces a layer of aluminium oxide on its surface which is impermeable to water, so it stops the reaction from proceeding further.
(ii) Add dilute hydrochloric acid to excess aluminium (or to aluminium oxide/hydroxide). Filter off the excess metal, leave the filtrate in a warm place (or evaporate the filtrate) to the point of crystallization, then leave it (e.g. in the sun) to crystallize.

(d) The flask containing the aqueous CuSO4 is connected to a condenser and the solution is heated. The water vaporizes and is converted back to liquid water in the condenser (simple distillation), leaving the CuSO4 behind.

WAEC Chemistry 2023 Theory — Question 4

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