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. `200.8`
B. `206.88`
C. `210.5`
D. `215.2`

Answer

B

Explanation

Relative lowering in vapour pressure,
`(p^(@)-p_s)/(p^(@))=(x_(2))/(x_(2)+x_(1))=(w//m)/(w/m+W/M)`
`(0.30mm)/(17.54mm)=(20//m)/(20//m+100//18)`
`rArrm=206.88`