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Depression in Freezing Point - Practice Questions & MCQ

Edited By admin | Updated on Sep 25, 2023 25:23 PM | #NEET

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  • Depression in Freezing Point is considered one of the most asked concept.

  • 29 Questions around this concept.

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

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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 Tof Tf 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 (ΔTf)
  • It can be measured by Beckmann's thermometer method and Rast's method.
  • For a dilute solution, ΔTis directly propotional to the molality (m) of the solution.
    Hence ΔTf ∝ m
              ΔTf = Km
    If molality of the solution is one, then
    ΔTf = Kf
    ΔTf 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. }

NOTE: The value of Kv or Kf depends on only nature of solvent and not on the nature of the solute.

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Depression in Freezing Point

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