1 mole of liquid A and 2 moles of liquid B make a solution having a total vapour pressure 40 torr. The vapour pressure of pure A and pure B are 30 torr and 45 torr respectively. The above solution :
Show Hint
- is an ideal solution.
- shows negative deviation from Raoult's Law.
- shows positive deviation from Raoult's Law.
- is a maximum boiling azeotrope.
The Correct Option is A
Solution and Explanation
Step 1: Find the mole fractions
Total moles $= 1 + 2 = 3$, so: \[ x_A = \tfrac{1}{3}, \qquad x_B = \tfrac{2}{3} \]
Step 2: Predict the pressure using Raoult's law
\[ P_{predicted} = x_A P_A^\circ + x_B P_B^\circ = \tfrac{1}{3}(30) + \tfrac{2}{3}(45) = 10 + 30 = 40\ \text{torr} \]
Step 3: Compare with the real value
The measured total pressure is also 40 torr, which is exactly equal to what we predicted. Since there is no difference at all, there is no deviation, so the solution is ideal.
Answer: Option (A), it is an ideal solution.
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