JEE Advanced 2026 Paper 2, Chemistry Section 4 Q2: Vapour Pressure Of A Solution

Question Stem (Q1-Q2). Two volatile liquids $\mathbf{A}$ and $\mathbf{B}$ form an ideal solution. Consider a $5$ molal solution of $\mathbf{B}$ in $\mathbf{A}$ inside a closed container having a total vapour pressure of $100$ mm Hg at $300$ K. The vapour pressure of pure $\mathbf{A}$ at $300$ K is $105$ mm Hg. Assume that $\mathbf{A}$ and $\mathbf{B}$ behave as ideal gases in the vapour phase.

Given: Gas constant $R=0.08$ L atm K${}^{-1}$ mol${}^{-1}$; molar mass of $\mathbf{A}$ is $50$ g mol${}^{-1}$; molar mass of $\mathbf{B}$ is $57$ g mol${}^{-1}$; density of liquid $\mathbf{B}$ at $300$ K is $0.5$ g/mL; $1$ atm $=760$ mm Hg.

Q2. The mole fraction of $\mathbf{B}$ in vapour phase which is in equilibrium with this solution is _____.