Solubility curves

3 marks · Normal

A solution contains 90 g of ammonium nitrate dissolved in 100 g of water at 60°C. The solubility curve for ammonium nitrate shows that at 60°C, the solubility is 90 g per 100 g water, and at 30°C the solubility is 55 g per 100 g water.

Explain what happens as the solution is cooled from 60°C to 30°C. Calculate the mass of ammonium nitrate that will crystallise out. Show your reasoning.

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As the solution cools to 30°C, only 55 g can stay dissolved. (90 - 55) = 35 g will crystallise out. g

Must show: States decrease in solubility on cooling, Calculation of mass crystallising out: 90 - 55 = 35 g

Also accept: 35; 35 g

Common mistakes: Adding instead of subtracting solubilities; Not showing working

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Marking points

M1 · 1 mark · States that solubility decreases with temperature (on cooling) or that less stays dissolved.

M2 · 1 mark · Identifies the mass left in solution at 30°C is 55 g.

A1 · 1 mark · Calculates mass crystallised: 90 - 55 = 35 g.

Worked solution

As the solution is cooled, the solubility of ammonium nitrate decreases. At 60°C all 90 g is dissolved. At 30°C, only 55 g remains dissolved.

So, 35 g of ammonium nitrate will crystallise out.