The vapor pressure of pure toluene is 40 mm Hg at a certain temperature. If a solution with a non-volatile solute has a
(p⁰ - p/p⁰) = xsolute . (40 - 38/40) = (2/40) = 0.05 .
Ref: NCERT Class 12 Chemistry > Chapter 1: Solutions > Topic: Types of Solutions and Expressing Concentration