NEET 2025 — Chemistry solution

NEET2025Chemistry

$5$ 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?

A
The solution shows positive deviation.
B
The solution shows negative deviation.
C
The solution is ideal.
D
The solution has volume greater than the sum of individual volumes.

Solution

Mole fractions: $x_{X} = 5/15 = 1/3$ ; $x_{Y} = 10/15 = 2/3$ .

Ideal (Raoult's law) prediction: $P_{c a l c} = P_{X}^{\circ} x_{X} + P_{Y}^{\circ} x_{Y} = 63 \times \frac{1}{3} + 78 \times \frac{2}{3} = 21 + 52 = 73$ torr.

Observed: $P_{o b s} = 70$ torr $< P_{calc}$. Since the actual vapour pressure is less than the ideal prediction, the solution shows negative deviation from Raoult's law (stronger A-B interactions than A-A and B-B).

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