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Physical Chemistry

Quick questions on Solubility product Ksp and the common ion effect: Singapore A-Level H2 Chemistry

5short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.

What is linking KspK_{sp} to solubility?
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Let molar solubility be ss. The ion concentrations follow the stoichiometry:
What is the common ion effect?
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Adding an ion already present in the equilibrium (a common ion) decreases the solubility of the salt. Because KspK_{sp} is fixed, raising one ion's concentration forces the other ion's concentration down, so less solid dissolves. This is a direct application of Le Chatelier to the solubility equilibrium, and it underlies selective precipitation in qualitative analysis.
What is q1?
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Write the KspK_{sp} expression for lead(II) chloride, PbCl2\text{PbCl}_2. [1 mark]
What is q2?
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The solubility of Mg(OH)2\text{Mg(OH)}_2 is 1.2×10−41.2 \times 10^{-4} mol dm−3^{-3}. Calculate its KspK_{sp}. [3 marks]
What is q3?
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Explain, using KspK_{sp}, why barium sulfate is safe to use as a medical "barium meal" despite barium ions being toxic. [2 marks]

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