100 g of liquid A (molar mass `140 g mol^(-1)`) was dissolved in 1000 g of liquid B (molar mass `180 g mol^(-1)`). The vapour pressure of pure liquid B was found to be 500 torr. Calculate the vapour pressure of pure liquid A and its vapour pressure in the solution, if the total vapour pressure of the solution is 475 torr.

Explanation

Number of moles of liquid A (solute)
`= (100g)/(140 g mol^(-1)) = (5)/(7) mol`
Number of moles of liquid B `= (1000 g)/(180 g mol^(-1)) = (50)/(9) mol`
`therefore` Mole fraction of A in the solution `(chi_(A))`
`= (5//7)/(5//7 + 50//9) = (5//7)/(395//63)`
`= (5)/(7) xx (63)/(395) = (45)/(395) = 0.114`