5 moles of liquid X and 10 moles of liquid Y make a solution having a vapour pressure of 70 torr. Th

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Question

5 moles of liquid X and 10 moles of liquid Y make a solution having a vapour pressure of 70 torr. The vapour pressures of pure X and Y are 63 torr and 78 torr respectively. Which of the following is true regarding the described solution?

Accepted Answer

Correct Answer: C — The solution shows negative deviation from Raoult's law.Step-by-step Solution:Mole fractions: XX = 5/15 = 1/3; XY = 10/15 = 2/3Ideal vapour pressure (Raoult's law):Pideal = XXP°X + XYP°Y = (1/3)(63) + (2/3)(78) = 21 + 52 = 73 torrObserved P = 70 torr ∴ Observed Negative deviation from Raoult's lawWhy other options are wrong:A → Volume > sum of individual volumes = positive deviation propertyB → Positive deviation: Pobs > Pideal (not the case here)D → Ideal: Pobs = Pideal = 73 torr (not 70 torr)Key Concepts:Raoult's law: Ptotal = X₁P°₁ + X₂P°₂ (for ideal solutions)Negative deviation: Pobs ideal → X–Y intermolecular forces stronger than X–X and Y–Y → ΔHmix mix Positive deviation: Pobs > Pideal → X–Y forces weaker → ΔHmix > 0; ΔVmix > 0. Example: ethanol–water.

5 moles of liquid X and 10 moles of liquid Y make a solution having a vapour pressure of 70 torr. The vapour pressures of pure X and Y are 63 torr and 78 torr respectively. Which of the following is true regarding the described solution?

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5 moles of liquid X and 10 moles of liquid Y make a solution having a vapour pressure of 70 torr. The vapour pressures of pure X and Y are 63 torr and 78 torr respectively. Which of the following is true regarding the described solution?

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