The vapour pressure of water at `20^(@)C is 17.54mm`. When 20g of non - ionic substance is dissolved in 100g of water, the vapour pressure is lowered by `0.30mm`. What is the molecular mass of the substance ?

Options

A. `210.2`
B. `206.88`
C. `215.2`
D. `200.8`

Answer

B

Explanation

`(P_(o)-P_(s))/(P_(s))=(w)/(m)xx(M)/(W)`
`P_(o)=17.54 mm`
`P_(s)=17.24 mm`
M = 18
W = 100
w = 20
m = ?
`(17.54-17.24)/(17.24)=(20)/(m)xx(18)/(100)`
`(30)/(17.24)=(360)/(100m)`
`m=(360xx17.24)/(100xx30)rArr206.88`