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…

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?

1. A. The solution shows positive deviation
2. B. The solution shows negative deviation (Correct answer)
3. C. The solution is ideal
4. D. The solution has volume greater than the sum of individual volumes

Correct Answer

Option B — The solution shows negative deviation

Detailed Solution & Explanation

The correct answer is the solution shows negative deviation.

Key Points

• For an ideal solution (Raoult's law): $P_{total}=x_XP^\circ_X+x_YP^\circ_Y=\dfrac{5}{15}(63)+\dfrac{10}{15}(78)=21+52=73$ torr.
• The observed vapour pressure (70 torr) is less than the ideal value (73 torr).
• A lower-than-ideal vapour pressure indicates stronger X-Y interactions, i.e. negative deviation from Raoult's law.