Question

3. (a) Determine the volume of water that should be added to 100 cm3 of 0.5 mol dm⁻³ HCl in order to obtain 0.3 mol dm⁻³ HCl.
(b) Describe briefly a chemical test to distinguish between dilute HCl and dilute HNO3. Support the test with a relevant equation.

Accepted Answer

(a) Using C1V1 = C2V2: 0.5 × 100 = 0.3 × V2, so V2 = (0.5 × 100)/0.3 = 166.67 cm3.
Volume of water to be added = 166.67 − 100 = 66.67 cm3.

(b) Add AgNO3(aq) to each solution in a separate test tube. Formation of a white precipitate indicates the solution is HCl; if no precipitate forms, the solution is HNO3.
AgNO3(aq) + HCl(aq) → AgCl(s)↓ + HNO3(aq)

(Alternative tests: (i) Heat each sample in a test tube — brown fumes indicate HNO3, no brown fumes indicates HCl. (ii) To each sample, add fresh FeSO4(aq), then carefully add concentrated H2SO4 down the side of the test tube — a brown ring at the interface indicates HNO3 (brown-ring test), no brown ring indicates HCl. FeSO4 + NO → FeSO4.NO)

WAEC Chemistry 2023 Theory — Question 14

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