Solubility curves
2 marks · EasyA solution contains 52 g of potassium iodide dissolved in 100 g of water at 60°C. The solubility curve for potassium iodide shows a solubility of 70 g per 100 g water at 60°C and 40 g per 100 g water at 30°C. Calculate the mass of potassium iodide that will crystallise out if the solution is cooled from 60°C to 30°C.
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12 g (±0.5)
Also accept: 12
Common mistakes: Forgetting to subtract new solubility from original dissolved mass
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Marking points
M1 · 1 mark · Finds how much will remain dissolved at 30°C (40 g per 100 g water).
A1 · 1 mark · Subtracts the lower solubility from the original mass: 52 – 40 = 12 g crystallise out.
Worked solution
At 30°C, only 40 g of potassium iodide will stay dissolved in 100 g water. Starting with 52 g, 52 – 40 = 12 g will crystallise out.