An ideal mixture of liquids A and B with 2 moles of A and 2 moles of B has a total vapour pressure of 1 atm at a certain temperature. Another mixture with 1 mole of A and 3 moles of B has a vapour pressure greater than 1 atm. But if 4 moles of C are added to the second mixture, the vapour pressure comes down to 1 atm. Vapour pressure of C, Pc° = 0.8 atm. Calculate the vapour pressures of pure A and pure B.
= 1 atm PA° + PB° = 2 atm
> 1 atmrArrPA° + 3PB° > 4 atm
and + = 1 atmrArrPA° + 3PB° + 4P C° = 8 atm
So PA° + 3PB° = (8 – 4 × 0.8 ) atm = 4.8 atm
Hence PB° = 1.4 atm
PB° = 0.6 atm