Two volatile liquids A and B form an ideal solution. Consider a 5 molal solution of B in A inside a closed container having a total vapour pressure of 100 mm Hg at 300 K. The vapour pressure of pure A at 300 K is 105 mm Hg. Assume that A and B behave as ideal gases in the vapour phase. Given: The gas constant R = 0.08 L atm K⁻¹ mol⁻¹; molar mass of A is 50 g mol⁻¹; molar mass of B is 57 g mol⁻¹; density of liquid B at 300 K is 0.5 g/mL; 1 atm = 760 mm Hg.
The mole fraction of B in vapour phase which is in equilibrium with this solution is ____ .