Question

the vapour pressure of two liquids A and B in their pure states are in ratio 1:2. A binary solution of A and B has A and B in mole proportion 1:2. The mole fraction of A in vapour phase of the solution will be ??

is the answer 0.33 or 0.2( because as the molefraction of A in vapour phase is used should we not use daltons law of partial pressure??)

Answer 1

Vapour pressure of liquid 1, P1 = 1units
Vapour pressure of liquid 2, P2 = 2 unit
Moles of liquid 1 = 1
Moles of liquid 2 = 2
Mole fraction of liquid 1 = 1/3
Mole fraction of liquid 2 = 2/3
Using Raoult's law:
Partial pressure of liquid 1 = Vapour pressure of liquid 1 mole freaction of liquid 1
Partial pressure of liquid 1 = 1 (1/3) = 1/3

Partial pressure of liquid 2 = ​Vapour pressure of liquid 2 mole freaction of liquid 2
Partial pressure of liquid 2 = 2(2/3) = 4/3

Total pressure p = partial pressure of 1 + partial pressure of 2 = 1/3+4/3=5/3

Mole fraction of liquid 1 in vapour phase = partial pressure of liquid1/total vapour pressure =( 1/3)/(5/3)=1/5 =0.2

Hope this clears your doubt.
Good luck.