The vapour pressure of a solvent decreased by 10 mm of Hg when a non-volatile solute was added to th — Solutions and Colligative Properties Chemistry Question

💡 Solution & Explanation

According to Raoult's law, $\Delta P = P^\circ X_{solute}$. From the first case, $10 = P^\circ(0.2)$, so $P^\circ = 50 \text{ mm Hg}$. In the second case, $\Delta P = 20 \text{ mm Hg}$. $20 = 50 \times X_{solute} \Rightarrow X_{solute} = 20/50 = 0.4$. The mole fraction of the solvent is $1 - X_{solute} = 1 - 0.4 = 0.6$. Therefore, correct answer is B.