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. `6.173m`
B. `3.0864m`
C. `1.543m`
D. `0.772m`

Answer

A

Explanation

`(Deltap)/(p^(@))=x_("solute")=(n_(1))/(n_(1)+n_(2))and(4)/(40)=(n_(1))/(n_(1)+n_(2))`
`n_(2)=9n_(1),(w_(2))/(m_(2))=9n_(1)andw_(2)=9n_(1)m_(2)`
`"Molality" =(1000w_(1))/(m_(1)w_(2))=(1000n_(1))/(w_(2))`
`=(1000n_(1))/(9n_(1)m_(2))=(1000)/(9xx18)=6.173 " molal"`