States of Matter
6 marks · NormalA student performs experiments to investigate how much of a solute dissolves in water, and how diffusion works in gases.
The solubility of potassium nitrate in water at 40°C is 64 g per 100 g of water.
Calculate the maximum mass of potassium nitrate that will dissolve in 250 g of water at this temperature.
In a diffusion tube experiment, the white ring of ammonium chloride forms 12 cm from the ammonia end and 8 cm from the hydrochloric acid end.
Calculate the ratio of speed of ammonia particles to hydrogen chloride particles.
State one reason why it is important to measure the water mass carefully in a solubility experiment.
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) 160 g
(b) The speed ratio is 12:8 = 1.5:1
(c) So that the calculated solubility is accurate and not too high or too low (since solubility depends on the mass of water used).
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Correctly sets up the proportional calculation (64 g per 100 g × 250 g)
A1 (a) · 1 mark · Accurate final answer: 160 g (allow error carried through for method) (follow-through)
M2 (b) · 1 mark · States that the ratio of distances equals the ratio of speeds
A2 (b) · 1 mark · Correctly calculates 12:8 (or simplifies to 3:2 or 1.5:1) (follow-through)
B1 (b) · 1 mark · Expresses speed ratio as 1.5:1, 3:2, or equivalent
B2 (c) · 1 mark · Gives a valid reason for measuring water mass accurately
Worked solution
a) Solubility is 64 g per 100 g of water. For 250 g water: g
b) The ratio of diffusion speed equals the ratio of distances travelled (since they start at the same time):
c) Because the calculated solubility depends on the exact mass of water used, so inaccurate measurement gives incorrect results.