The vapour pressures of pure liquids A and B are 400 and 600 mm Hg respectively at 298 K. On mixing the two liquids,…

Strategy:\n> Use Raoult's Law to find the total pressure ($P_T = x_A P_A^\circ + x_B P_B^\circ$) and then apply Dalton's Law ($y_i = P_i / P_T$) to find the vapour phase mole fractions.\n\nStep 1: Identify the solution type\n"Sum of volumes... equal to... final mixture" $\implies \Delta V_{\\text{mix}} = 0$, indicating an ideal solution.\n\nStep 2: Calculate Total Vapour Pressure ($P_{\\text{total}}$)\n$x_A = 0.5, x_B = 0.5$.\n$P_{\\text{total}} = (0.5 \\times 400) + (0.5 \\times 600) = 200 + 300 = 500\\text{ mm Hg}$\n\nStep 3: Calculate vapour phase composition\n- $y_A = \\frac{P_A}{P_{\\text{total}}} = \\frac{200}{500} = 0.4$\n- $y_B = \\frac{P_B}{P_{\\text{total}}} = \\frac{300}{500} = 0.6$\n\n$\boxed{\\text{Answer: (4)}}$