Numerical

Write the formula to determine the molar mass of a solute using freezing point depresssion method.

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#### Solution

Depression of freezing point and molar mass of the solute:

When W2 gram of solute with molar mass M2 is dissolved in W1 gram of solvent, the molality m is given by

`ΔT_f ∝ m`

`m="Mass of solute in kg"/("Molar mass of solue in kg "×" Mass of solvent in kg")`

`m=W_2/(M_2*W_1)`

∴ The molar mass of a solute using freezing depression method is givenby,

`ΔT_f ∝ m`

`ΔT_f = k_f . ("W"_2)/("M"_2) xx 1000/"W"_1`

Where `K_f` - Cryoscopic constant

`"W"_2` - mass of solute

`"W"_1` - mass of solvent

`"M"_2` - molecular mass of non-volatile solute

Concept: Colligative Properties and Determination of Molar Mass - Depression of Freezing Point

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