Calculations involving Ksp
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Calculating Ksp
Some students find calculations involving Ksp to be quite challenging. You can help students by providing 'scaffolding' to begin with. One way of doing this is to design a set of cards which students have to put in sequence to obtain the answer.
Let's look at a calculation to calculate the solubility product of magnesium hydroxide, Mg(OH)2, given its solubility at 298 K is 1.71 x 10–4 mol dm–3. A suitable set of cards might look like this:
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Which set of cards need to be placed together to show the equilibrium equation?
Cards 1, 4, 2 and 7.
Mg(OH)2(s) Mg2+(aq) + 2OH–(aq)
Which set of cards need to be placed together to show the expression for Ksp?
Cards 5, 9 and 11.
Ksp [Mg(OH)2(s)] = [Mg2+(aq)][OH–(aq)]2
Which set of cards need to be placed together to show how the value of Ksp is calculated?
Cards 8 and 12.
[Mg2+(aq)] = 1.71 x 10–4 mol dm–3
Cards 10 and 13.
[OH–(aq)] = 2 x 1.71 x 10–4 = 3.42 x 10–4 mol dm–3
Cards 6, 15, 14 and 3.
Ksp [Mg(OH)2(s)] = (1.71 x 10–4) x (3.42 x 10–4)2 = 2.00 x 10–11 mol3 dm–9
Calculating solubilities
Calculating solubilities to quantify the effect of a common ion is more demanding and a worked example will help students appreciate how to tackle the problem. This is illustrated in the following animation in which the solubility of barium sulfate in water and in sodium sulfate solution is compared.
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How many times less soluble is barium sulfate in a solution of sodium sulfate than it is in water?
The barium sulfate is 10–5/10–9 = 104 or 10,000 times less soluble in the sodium sulfate solution.