FRQ — Equilibrium

Long answer·Hard exam skill (4/5)·Calculator allowed

Difficulty guide

The label describes the problem, not your ability.

1/5 - Foundational

Warm-up setup; checks one basic idea.

2/5 - Routine exam skill

Normal syllabus skill; usually one main rule or interpretation.

3/5 - Standard multi-step

Exam-style problem with multiple steps or a common trap.

4/5 - Hard exam skill

High-end exam problem; combines methods carefully.

5/5 - Challenge

Stretch problem beyond normal exam pressure.

Question

A solution contains

0.010\,\mathrm{M}\ \mathrm{Ba^{2+}}

and

0.010\,\mathrm{M}\ \mathrm{Ca^{2+}}

. Sulphate ions are slowly added. Given

K_{sp}(\mathrm{BaSO_4})=1.0\times10^{-10}

and

K_{sp}(\mathrm{CaSO_4})=2.4\times10^{-5}

, compare which sulphate precipitates first.

Total: 5 marks

(a) Find $[\mathrm{SO_4^{2-}}]$ needed to start $\mathrm{BaSO_4}$ precipitation. (2 marks)

(b) Find $[\mathrm{SO_4^{2-}}]$ needed to start $\mathrm{CaSO_4}$ precipitation. (2 marks)

Your response (optional — you can also work on paper)

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