Question

2022 CHEMISTRY PRACTICAL QUESTIONS
1. A is an aqueous solution of Iodine. B is 0.100 mol dm⁻³ sodium trioxothiosulphate (VI).
(a) Put B into the burette. Pipette 20.0 cm³ or 25.0 cm³ of A into a conical flask. Add B from the burette until the reddish-brown colour fades to pale yellow, then add a few drops of starch indicator to obtain a dark blue solution. Continue adding B slowly from the burette until one drop of B causes the blue colour to disappear, leaving a colourless solution.
Repeat the titration to obtain concordant titre values. Tabulate your results and calculate the average volume of B used.
The equation for the reaction is: 2Na2S2O3(aq) + I2(aq) → Na2S4O6(aq) + 2NaI(aq)
(b)(i) From your results and the information provided, calculate the concentration in mol dm⁻³ of iodine in A;
(ii) Given a reason for each of the answers stated in 3b(i).
(c) State the method used in separating each of the following mixtures: (i) two miscible liquids; (ii) soluble salt and insoluble salt.
(d) Explain briefly why a solution of KCl does not give off a gas when mixed with NaHCO3 solution, but a solution of AlCl3 does.

Accepted Answer

(a) Sample titration results (illustrative, as recorded):
Burette readings (cm³): 1st Titre 25.60→1.20=24.40; 2nd Titre 32.80→8.70=24.10; 3rd Titre 26.40→2.30=24.10.
Average volume of Na2S2O3 used = (24.10+24.10)/2 = 24.10 cm³ (using the two concordant titres).

(b)(i) Given: CB = 0.1 mol dm⁻³ (B), VB = 24.10 cm³, VA = 25 cm³ (volume of pipette used), CA = ?
From the equation 2Na2S2O3 + I2 → Na2S4O6 + 2NaI: nA (I2) = 1, nB (Na2S2O3) = 2.
Using CAVA/nA = CBVB/nB: CA×25/1 = 0.1×24.10/2 → CA = (0.1×24.10×1)/(25×2) = 0.0482 mol dm⁻³.

(ii) Since iodine (I2) is diatomic, this must be taken into account when calculating molar mass — the molar mass of iodine used is 2×127 = 254 g/mol, not 127 g/mol. The subscripts 'A' and 'B' refer to whichever solution is assigned as acid/base equivalent in the mole ratio calculation — any two solutions can be taken as 'A' and 'B'.

(c)(i) Method for separating two miscible liquids: Fractional distillation.
(ii) Method for separating soluble salt and insoluble salt: Dissolution followed by filtration.

(d) While a solution of NaHCO3 and KCl might swap ions, there would be no visible reaction because mixing will not visibly change state (no gas evolved) — KCl and NaHCO3 do not react to release CO2 as no insoluble product/effervescence-producing reaction occurs. The reaction with AlCl3, however, does produce effervescence due to the formation of CO2 gas: 3NaHCO3 + AlCl3 → Al(OH)3 + 3NaCl + 3CO2.

WAEC Chemistry 2022 Theory — Question 8

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