Solubility curves
2 marks · EasyExplain, using a solubility curve, how you would determine how much solid potassium sulfate would crystallise from a saturated solution as it cools from 70°C to 30°C.
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Subtract the solubility at 30°C from the solubility at 70°C (for 100 g water); this difference is the mass of potassium sulfate that will crystallise out as the solution cools.
Must show: Reference to solubility at each temperature, Subtracts lower solubility from higher solubility to find mass crystallised
Also accept: Describe finding the mass of solute that exceeds the lower temperature's solubility and stating that this is the mass that crystallises
Common mistakes: Switching the subtraction order; not mentioning per 100 g water
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Marking points
M1 · 1 mark · Identifies that you find solubility at both temperatures from the curve.
M2 · 1 mark · Subtracts solubility at the lower temperature from the higher temperature to get the mass crystallised.
Worked solution
Find the solubility at 70°C from the curve, and then at 30°C. Subtract the solubility at 30°C from that at 70°C. The difference is the mass of potassium sulfate that crystallises out as the temperature falls.