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A-Level Solubility product and the common ion effect
What the A-Level syllabus expects for Solubility product and the common ion effect, and how to practise it.
What the syllabus expects
- Grasp the idea of solubility product, Ksp, and put it to use.
- Move between ion concentrations and Ksp in both directions.
- Discuss how each of the following changes the solubility of an ionic salt:
- the common ion effect
- the making of a complex ion, shown when halide ions meet aqueous silver ions and then aqueous ammonia
Scope: see also Section 13
How it's examined
Questions on this topic most often ask you to calculate. About 4% of the past-paper style questions in Rae's bank for this subject sit in this topic.
Worked examples
Example 1 (2 marks)
H2S is added to another 1 dm3 sample containing 1.0e-9 mol dm-3 each of Hg2+ and Pb2+. Using Ksp(PbS) = 9e-29 and Ksp(HgS) = 2e-53 mol2 dm-6, calculate the minimum [H2S] that removes the most Hg2+ without precipitating Pb2+, and hence the maximum mass of HgS formed in 1 dm3.
Show the worked answer
To avoid precipitating PbS, keep [Pb²+][S²-] below Ksp(PbS). With [Pb²+] = 1.0e-9, the maximum sulfide concentration is [S²-] = Ksp(PbS)/[Pb²+] = 9e-29 / 1.0e-9 = 9e-20 mol dm⁻³. At this sulfide level almost all Hg²+ is removed: residual [Hg²+] = Ksp(HgS)/[S²-] = 2e-53 / 9e-20 = 2e-34 mol dm⁻³ (negligible). So essentially all 1.0e-9 mol of Hg²+ in 1 dm3 precipitates as HgS. M(HgS) = 200.6 + 32.1 = 232.7 g mol⁻¹, so maximum mass = 1.0e-9 x 232.7 = 2.3e-7 g.
More A-Level H2 Chemistry topics
The make-up of the atom and how its electrons are arranged · How atoms bond and how that governs a substance's behaviour · Ideal gases and working with gas mixtures · Competing definitions of acids and bases · Trends in the elements across a period and down a group · The mole and reacting-quantity calculations · all of A-Level H2 Chemistry