The mass of a non-volatile solute (molecular mass = 40) which should be dissolved in 114 g octane to — Solutions and Colligative Properties Chemistry Question

💡 Solution & Explanation

Reducing the vapour pressure to 80% means P / P^o = 0.8. According to Raoult's law: P / P^o = X_solvent = N / (n + N). Octane (C8H18) has a molar mass of 114 g/mol. Moles of octane N = 114 / 114 = 1.0 mol. Substituting into the equation: 0.8 = 1 / (n + 1) => 0.8n + 0.8 = 1 => 0.8n = 0.2 => n = 0.25 mol. Mass of solute = n * molar mass = 0.25 * 40 = 10 g.