Pure water freezes at 273 K and 1 bar. The addition of 34.5 g of ethanol to 500 g of water changes the freezing point of the solution. Use the freezing point depression constant of water as 2 K `kg mol^(-1).` The figures shown below represent plots of vapour pressure (V.P.) versus temperature (T). [molecular weight of ethanol is` 46 g mol^(-1)` Among the following, the option representing change in the freezing point is

Options

A.
B.
C.
D.

Answer

D

Explanation

For depression in freezing point :
`DeltaT=K_(f)xx(w_(B)xx1000)/(m_(B)xxw_(A))`
`=2xx(34.5xx1000)/(46xx500)=3`
`T_(0)-T=3`
`273-T=3`
`:. T=270K`
The graph in option (d) is correct which shows lowering of vapour pressure with temperature and
`DeltaT=(T_(0)=T)=(273-270)=3`].