The mole fraction of B in vapour phase which is in equilibrium with this solution is .
Answer
0.16
Solution
5 molal: 5 mol B in 1000 g A (=20 mol A), so xB= 525=0.2, xA=0.8
P=xApA +xBpB 100=0.8 105+0.2 pB pB =80 mm Hg
Dalton's law: yB= xBpB P= 0.2 80100=0.16
Answer: 0.16