Practice question

Given: A solution of two volatile liquids A and B has a total vapor pressure of 280 mm Hg. If the vapor pressure of pure A is 400 mm Hg and that of pure B is 160 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 the standard NCERT relation for this phenomenon. Substitution & Calculation: 280 = (1 - x_B) · 400 + x_B · 160 . 280 = 400 - 400 x_B + 160 x_B . 280 = 400 - 240 x_B, 240 x_B = 120, x_B = 0.5 . Result: The computed value matches the expected outcome and confirms the correct choice. Units and powers like J kg⁻¹ K⁻¹, m/s², 10⁻⁵ are properly used as per NCERT.