Principles of Chemistry
8 marks · NormalA mixture of potassium nitrate and sand is supplied to a student. She performs a series of experiments to separate the components and calculate solubility.
Describe the two separation techniques needed to obtain pure, dry potassium nitrate from the mixture.
The student adds the mixture to 40.0 g of water and stirs until no more potassium nitrate dissolves. After filtration, she evaporates the solution to recover 22.8 g of dry potassium nitrate. Calculate the solubility of potassium nitrate in water at this temperature (in g per 100 g water).
If the solubility of potassium nitrate at 60°C is 110 g per 100 g of water, and at 30°C it is 45 g per 100 g water, calculate the mass of potassium nitrate that will crystallise out when a saturated solution containing 110 g of potassium nitrate in 100 g water at 60°C is cooled to 30°C.
State why it is important to use filtration after dissolving the mixture in water.
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) — · must show Filtration, Crystallisation or evaporation
(b) 57.0 g per 100 g water · must show 22.8/40.0 × 100
(c) 65 g · must show 110 – 45
(d) — · must show To remove insoluble sand
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Names filtration as first separation step.
M2 (a) · 1 mark · Names crystallisation or evaporation (or correct explanation) as second separation step.
M3 (b) · 1 mark · Uses and sets up solubility formula: (mass of solute/mass of water) × 100.
A1 (b) · 2 marks · Correct calculation: (22.8/40.0) × 100 = 57.0 g per 100 g water.
M4 (c) · 1 mark · Correctly subtracts lower solubility from upper to find mass of crystals.
A2 (c) · 1 mark · Calculates 110 – 45 = 65 g.
M5 (d) · 1 mark · States filtration removes insoluble material (sand) from solution.
Worked solution
a. First, filter the mixture to separate insoluble sand from the solution of potassium nitrate. Then evaporate the water or leave to crystallise, and collect dry potassium nitrate.
b. Solubility = (mass of solute / mass of water) × 100 = (22.8 / 40.0) × 100 = 57.0 g per 100 g water.
c. When cooled from 60°C to 30°C, the mass that will crystallise is 110 – 45 = 65 g.
d. Filtration after dissolving removes insoluble sand so that only the dissolved potassium nitrate is included in further steps.