Question

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

• A.
• B.
• C.
• D.

Answer 1

The correct option is D

$\Delta T_f=i{K}_f\left[\frac{W_2\times 1000}{M_2\times W_1}\right]$

$=1\times 2\left[\frac{34.5\times 1000}{46\times 500}\right]=3K$

$273\left(K\right)-T_f=3\left(K\right)$
$\Rightarrow T_f=270K$
Also, with decrease in temperature, V.P. decreases.
$\therefore$ Graph (3) is correct.