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

B

Explanation

As T increase , the vapour pressure increases.
Therefore , options (a) and (d) are not correct.
`DeltaT_(f) = (k_(f) xx w_(B) xx 1000)/(M_(B) xx w_(B))`
` = ( 2 xx 34.5 xx 1000)/(46 xx 500) = 3K`
`Delta T_(f) = 273(K) - T_(f) = 3K`
`Delta T_(f) = 273(K) - T_(f) = 3K`
or `" "T_(f) = 273 - 3 = 270 K`
Therefore , option (b) is correct .