Two liquids A and B form an ideal solution. At 320 K, the vapour pressure of the solution, containing 3 mol of A and 1 mol of B is 500 mm Hg. At the same temperature, if 1 mol of A is further added to this solution, vapour pressure of the solution increases by 20 mm Hg. Vapour pressure (in mm Hg) of B in pure state is 
. (Nearest integer)
Text Solution
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For an ideal solution, Raoult's law states:
.
Initial condition: 3 mol of A and 1 mol of B give a total vapor pressure of 500 mm Hg.
Mole fractions are 
and 
.
This gives: 
…… . (i)
After adding 1 mol of A : 4 mol of A and 1 mol of B give a total vapor pressure of 520 mm Hg.
Mole fractions are 
and 
.
This gives: 
……… .. (ii)
From equation (i) multiplied by 4: 
.
From equation (ii) multiplied by 5: 
.
Subtracting: 
.
Substituting back into equation (i): 
, so 
.
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