Question

What is the order of depression in freezing point of aqueous solution if same amount of acetic acid, trichloroacetic and trifluoroacetic acid are added in given amount of water?

• A. $\text{Acetic acid < Trichloroacetic acid < Trifluoroacetic acid}$
• B. $\text{Trifluoroacetic acid > Acetic acid > Trichloroacetic acid}$
• C. $\text{Trichloroacetic acid > Trifluoroacetic > Acetic acid}$
• D. $\text{Acetic acid > Trichloroacetic acid > Trifluoroacetic acid}$

Answer 1

The correct option is A $\text{Acetic acid < Trichloroacetic acid < Trifluoroacetic acid}$

Freezing point depression of a solution is given by,
$△T_f=i\times K_f\times m$
where,
$△T_f$ is the depression in freezing point.
$K_f$ is molal depression constant.
m is molality of the solution.
i is van't Hoff factor

The freezing point of water for the same amount of given compounds can be explained on the basis of the degree of ionisation, which depends upon the strength of the acid.

Trifluoroacetic acid is more acidic than trichloroacetic acid which is furthermore acidic than acetic acid.

Therefore the degree of ionisation of these acids will decrease in the following order:

$\text{Trifluoroacetic acid > Trichloroacetic acid > acetic acid}$

Now, greater the degree of ionisation, greater will be the depression of freezing point.

Hence, option A is correct.