The vapour pressure of a `5%` aqueous solution of a non-volatile organic substance at `373 K`. Is `745 mm`. Calculate the molecular mass of the solute.

Explanation

A 5% aqueous solution means 5 g of solute in 100 g of solution.
Mass of solution (water)=100-5=95g
Vapourt pressure of solution at 373 K = 745 mm
Vapour pressure of water at 373 K 760 mm
According to Raoult's Law, `=(p_(A)^(@)-P_(S))/P_(S)=n_(B)/n_(A)=W_(B)/M_(B)xxM_(A)/W_(A)`
`((760mm-745mm))/((745mm))=(%5gxx(18g mol^(-1)))/(M_(B)xx(95g))`
`15/745=((5g)xx(18gmol^(-1)))/(M_(B)xx(95g))`
`M_(B)=((5g)xx(18gmol^(-1))xx745)/((95g)xx15)=47 g mol^(-1)`