Two highly volatile liquids, A and B, form a completely ideal solution. At , the isolated vapour pre — Solutions and Colligative Properties Chemistry Question
Question
Two highly volatile liquids, A and B, form a completely ideal solution. At $300\text{ K}$, the isolated vapour pressure of pure liquid A is $100\text{ torr}$ and the vapour pressure of pure liquid B is $300\text{ torr}$. What is the total equilibrium vapour pressure (in torr) of a solution prepared by uniformly mixing $2\text{ moles}$ of A and $3\text{ moles}$ of B at $300\text{ K}$?
Answer: 220
💡 Solution & Explanation
The total moles in the mixture = $2 + 3 = 5$. Thus, the mole fraction $X_A = \frac{2}{5} = 0.4$ and $X_B = \frac{3}{5} = 0.6$. The total pressure $P_{total}$ is the sum of partial pressures: $P_{total} = (P_A^0 \times X_A) + (P_B^0 \times X_B) = (100 \times 0.4) + (300 \times 0.6) = 40 + 180 = 220\text{ torr}$.
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