Vapour pressure of pure water is 40 mm. if a non-volatile solute is added to it, vapour pressure falls by 4 mm. Hence, molality of solution is

Options

A. `6.173 m`
B. `3.0864 m`
C. `1.543 m`
D. `0.712 m`

Answer

A

Explanation

`(DeltaP)/(P^(@))=X_("solute")=(n_(1))/(n_(1)+n_(2))`
`(4)/(40)=(n_(1))/(n_(1)+n_(2))`
`n_(2)=9n_(1)(w_(2))/(m_(2))=9n_(1)`
`w_(2)=9n_(1)m_(2)`
Molality `=(100w_(1))/(m_(1)w_(2))=(1000n_(1))/(w_(2))`
`=(100m_(1))/(9n_(1)m_(2))=(1000)/(9xx18)=6.73` molal.
Hence, (A) is the correct answer.