Question

What should be the freezing point of aqueous solution containing $17g$ of $C_2{H}_{5}OH$ in $1000g$ of water ($K_f$ for water = ${1.86}^{\circ }Ckgmo{l}^{-1}$

• A. $-{0.69}^{\circ }C$
• B. ${0.34}^{\circ }C$
• C. ${0.0}^{\circ }C$
• D. $-{0.34}^{\circ }C$

Answer 1

The correct option is A $-{0.69}^{\circ }C$

Freezing point depression of a solution is given by,
$△T_f=K_f\times m$
where,
$△T_f$ is the depression in freezing point.
$K_f$ is molal depression constant.
m is molality of the solution.

$\text{Molality (m)}=\frac{\frac{w_{\text{solute}}}{M}\times 1000}{W_{\text{solvent}}}$
where,
$w$ is weight of solute
$W$ is weight of solvent
$M$ is molar mass of the solute
$△T_f=K_f\times \frac{w\times 1000}{M\times W}=1.86\times \frac{17\times 1000}{46\times 1000}=0.69$
Freezing point of pure water = $0^{\circ }C$
$\therefore$
Freezing point of solution $=0-0.69=-{0.69}^{\circ }C$