Solubility curves
4 marks · NormalA solution contains 60.0 g of magnesium sulfate completely dissolved in 150 g of water at 70°C. The solubility curve for magnesium sulfate: at 70°C, solubility = 44 g/100 g water; at 30°C, solubility = 30 g/100 g water.
Is the solution at 70°C saturated or unsaturated? Explain your answer.
If the solution is cooled to 30°C and no water evaporates, calculate the maximum mass of magnesium sulfate that remains dissolved.
Calculate the mass of magnesium sulfate that will crystallise out as the solution cools from 70°C to 30°C.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Unsaturated; 66 g can dissolve, only 60 g is present. · must show Comparison with solubility at 70°C
(b) 45 g · must show Calculation based on 30 g/100 g water in 150 g water (30/100 × 150)
(c) 15 g · must show 60 g initially - 45 g remaining = 15 g crystallises out
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Uses solubility value at 70°C for 150 g water: 44/100 × 150 = 66 g would dissolve.
A1 (a) · 1 mark · States solution is unsaturated, as 60 g < 66 g.
A2 (b) · 1 mark · Correctly calculates: 30/100 × 150 = 45 g remains dissolved at 30°C.
A3 (c) · 1 mark · Correctly calculates: 60 - 45 = 15 g crystallises out. (follow-through)
Worked solution
a) At 70°C, the solubility in 150 g water is (44 × 150/100) = 66 g. Only 60 g is dissolved, so the solution is unsaturated.
b) At 30°C, the solubility in 150 g water is (30 × 150/100) = 45 g.
c) Mass crystallised = Original dissolved - Remain dissolved = 60 - 45 = 15 g.