A very dilute saturated aqueous solution of a sparingly soluble salt `X_3 Y_4,` has a vapour pressure of 20 mm Hg at temperature T, while pure water exerts a pressure of 20.0126 mm Hg at the same temperature. Calculate molality (m) at temperature T:

Options

A. `6.3 xx 10^(-4)`
B. `3.5 xx 10^(-2)`
C. `5 xx 10^(-3)`
D. `8.2 xx 10^(-2)`

Answer

C

Explanation

`m= (P^(@)-P)/(P) xx (1000)/(M_("solvent")), 7m = ((20.0126- 20)xx 1000)/(20 xx 18)`