At a given temperature, the vapour pressure in mm of Hg. of a solution of two volatile liquids A and B is given by the equation :
`rho = 120-80 X_(B)`, `X_(B) =` mole fraction of B.
Vapour pressures of pure A and B at the same temperature are respectively

Explanation

When `X_(B)=0`, we have pure A`" "therefore" "p_(A)^(@)=120-80xx0=120mm`
The given expression can be written as `" "p=120-80(1-X_(A))`
When `X_(A)=0`, we have pure B`" "therefore" "p_(B)^(@)=120-80(1-0)=40mm`
Alternatively, for pure B, `x_(B)=1`. Hence, `p_(B)^(@)=120-80xx1=40mm`