Practice question

Given: A solution of two volatile liquids A and B has a total vapor pressure of 320 mm Hg. If the vapor pressure of pure A is 400 mm Hg and that of pure B is 200 mm Hg, what is the mole fraction of B in the solution? These values define the system as per NCERT data. Formula: Using Raoult's law: p_{total = x_A p_A⁰ + x_B p_B⁰, where x_A + x_B = 1. This is standard NCERT relation. Substitution & Calculation: 320 = (1 - x_B) · 400 + x_B · 200 . 320 = 400 - 400 x_B + 200 x_B . 320 = 400 - 200 x_B, 200 x_B = 80, x_B = 0.4 . Result: The computed value matches expected outcome and confirms correct choice as per NCERT.