JEE Main 2020 · 7 January · morning · Single correct · +4 / −1
At 35ᵒC, the vapour pressure of CS₂ is 512 mm. Hg and that of acetone is 344 mm Hg. A solution of CS₂ in acetone has a total vapour pressure of 600 mm Hg. The false statement amongst the following is :
- (a) CS₂ and acetone are less attracted to each other than to themselves.
- (b) a mixture of 100 ml. CS₂ and 100 ml. acetone has a volume < 200 ml.
- (c) Heat must be absorved in order to produce the solution 35ᵒC
- (d) Raoult's law is not obeyed by this system.
Answer and solution
Answer (b) a mixture of 100 ml. CS₂ and 100 ml. acetone has a volume < 200 ml.
Solution
Chemistry ma’am · 3 steps
600 mm is above even pure CS₂, so this is a strong positive deviation. Check each option against that.
- Read the pressure. For an ideal solution, the total vapour pressure lies between the two pure values, 344 and 512 mm Hg. Here it is 600 mm Hg, higher than even the more volatile liquid. So the system shows a large positive deviation from Raoult's law.
- What positive deviation means. Molecules escape more easily from the mixture than from the pure liquids. That happens when CS₂–acetone attractions are weaker than CS₂–CS₂ and acetone–acetone attractions. Weaker attractions give Δ Hₘᵢₓ > 0 and Δ Vₘᵢₓ > 0.
Option What it claims For positive deviation Verdict (a) Unlike molecules attract less than like ones Yes, that is the cause true (b) Final volume is less than 200 mL Δ Vₘᵢₓ > 0, so the volume is more than 200 mL false (c) Heat is absorbed on mixing Δ Hₘᵢₓ > 0, endothermic true (d) Raoult's law is not obeyed Any deviation means non-ideal true - So. The weaker attractions let the molecules spread out, so the mixture expands. The false statement is (b).
The idea to keep. Positive deviation means weaker unlike attractions, so mixing absorbs heat and the volume increases. Negative deviation is the exact mirror: heat released and volume shrinks.
Answer from the NTA final answer key. Solution by PrepWiser.