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A2 · Practice questions · Solubility product and the common ion effect

Will it precipitate?

Six original Cambridge-style questions on solubility products.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Ions only, powers from the formula.

01
Recall
[3 marks]

Write Ksp expressions, with units, for (a) BaSO4, (b) PbI2, (c) Ag2CrO4.

  • (a) [Ba2+][SO42−], mol2 dm−6. ✓
  • (b) [Pb2+][I−]2, mol3 dm−9. ✓
  • (c) [Ag+]2[CrO42−], mol3 dm−9. ✓
02
Calculation
[2 marks]

The solubility of calcium fluoride is 2.05 × 10−4 mol dm−3. Calculate Ksp.

  • [Ca2+] = 2.05 × 10−4, [F−] = 4.10 × 10−4 mol dm−3. ✓
  • Ksp = 2.05 × 10−4 × (4.10 × 10−4)2 = 3.45 × 10−11 mol3 dm−9. ✓
03
Calculation
[2 marks]

Ksp for BaSO4 is 1.08 × 10−10 mol2 dm−6. Calculate its solubility in mol dm−3 and in g dm−3 (Mr 233.4).

  • s = √(1.08 × 10−10) = 1.04 × 10−5 mol dm−3. ✓
  • Mass: 1.04 × 10−5 × 233.4 = 2.43 × 10−3 g dm−3. ✓
04
Calculation
[3 marks]

Ksp for PbI2 is 8.49 × 10−9 mol3 dm−9. Calculate its solubility in (a) water and (b) 0.100 mol dm−3 KI.

  • (a) 4s3 = 8.49 × 10−9, so s = 1.29 × 10−3 mol dm−3. ✓
  • (b) [I−] = 0.100 (the dissolved PbI2 adds almost nothing), so [Pb2+] = 8.49 × 10−9 ÷ 0.1002. ✓
  • s = 8.49 × 10−7 mol dm−3, about 1500 times less: the common ion effect. ✓
05
Analysis
[3 marks]

Equal volumes of 2.00 × 10−4 mol dm−3 AgNO3 and 2.00 × 10−4 mol dm−3 NaCl are mixed. Ksp(AgCl) = 1.80 × 10−10 mol2 dm−6. Will a precipitate form?

  • After mixing, [Ag+] = [Cl−] = 1.00 × 10−4 mol dm−3. ✓
  • Ionic product = 1.00 × 10−4 × 1.00 × 10−4 = 1.00 × 10−8 mol2 dm−6. ✓
  • This is greater than Ksp, so AgCl precipitates. ✓
06
Explain
[2 marks]

Explain why silver chloride is less soluble in sodium chloride solution than in pure water.

  • AgCl(s) ⇌ Ag+(aq) + Cl−(aq); NaCl adds a common ion, Cl−. ✓
  • The equilibrium shifts left to keep [Ag+][Cl−] equal to Ksp, so less AgCl can dissolve. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.