Depression in Freezing Point - Practice Questions & MCQ

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Depression in Freezing Point is considered one of the most asked concept.
27 Questions around this concept.

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What will happen when $\mathrm{NaCl}$ is added to aqueous solution

No Change in freezing point

Boiling point lowered

No change in boiling point

freezing point lowered

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Choose the correct statement about freezing point:

It decreases on increasing amount of solute

Nothing happens when amount added in solution

It increases on increasing amount of solute

It lowered on adding more solvent

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Concepts Covered - 1

Depression in Freezing Point

Freezing Point: It is the temperature at which the liquid and the solid form of the same substance are in equilibrium and have the same vapour pressure. A Solution freezes when it's vapour pressure is equal to the V.P of pure solid solvent. Due to lower vapour pressure of the solution, solid form of a solution separates out a lower temperature.
On adding a non-volatile solute to the solvent it's vapour pressure decreases and becomes equal to the V.P. of solid solvent at lower temperature hence the freezing point of solvent also decreases.
Suppose T^o_f T_f are the freezing point of pure solvent and solution respectively than decrease in freezing point ΔT is given as:
$\Delta \mathrm{T}_{\mathrm{f}}=\mathrm{T}_{\mathrm{f}}^{\circ}-\mathrm{T}_{\mathrm{f}}$

This is also termed as cryoscopy and depression of freezing point (ΔT_f)
It can be measured by Beckmann's thermometer method and Rast's method.
For a dilute solution, ΔT_fis directly propotional to the molality (m) of the solution.
Hence ΔT_f ∝ m
ΔT_f = K_fm
If molality of the solution is one, then
ΔT_f = K_f
ΔT_f and M can be found out by using these relations.
$\\\mathrm{\Delta T_{f}=K_{f}\: \frac{w}{M} \times \frac{1000}{W}}\\\\\mathrm{M=\frac{K_{f} \times w \times 1000}{\Delta T_{f} \times W}}$

$\text { Here } w=\text { Weight of solute }$
$\begin{array}{l}{\mathrm{W}=\text { Weight of solvent }} \\ {\mathrm{K_{f}}=\text { Molal depression constant or cryoscopic }} {\text {constant }} \\ {\mathrm{M}=\text { Molar mass of Non-volatile solute. }}\end{array}$
$\mathrm{K}_{\mathrm{f}}=\frac{\mathrm{RT}^{2}}{1000 \mathrm{L}_{\mathrm{f}} \text { or } \Delta \mathrm{H}_{\text {fusion }}}$
$\text { Here } \mathrm{L}_{f} \text { or } \Delta \mathrm{H}_{\mathrm{f}}=\text { latent heat of fusion. }$