1gm of non-electrolyte solute disolved in 50gm of benzene lowered the freezing point of benzene by 0.4 K. Freezing point depression constant of benzene is 5.12 K Kg/mol. Molar mass of solute is  

  • Option 1)

    252 g/mol

  • Option 2)

    256g/mol

  • Option 3)

    262 g/mol

  • Option 4)

    356 g/mol

 

Answers (1)
P Prateek Shrivastava

As we discussed in concept

Mathematical Expression of Depression in Freezing point -

\Delta T_{f}= K_{f}\: m
 

- wherein

m = molarity of solvent 

K_{f} = cryoscopic  constant

    molal depress const

Units = \frac{K-K_{g}}{mole}

 

 We know that \Delta T_{P}=K_{P}m

0.4 = 5.12 x m

where m = molality of solute

m=\frac{0.4}{5.12}

 molality=\frac{weight}{\frac{molecular weight}{weight of solvent in kg}}=

\frac{0.4}{5.12}\:=\:\frac{1}{molecular weight\times 0.05}

Molecular weight = \frac{5.12}{0.05\times 0.4} = 256\:g\:mol^{-1}


Option 1)

252 g/mol

This option is incorrect.

Option 2)

256g/mol

This option is correct.

Option 3)

262 g/mol

This option is incorrect.

Option 4)

356 g/mol

This option is incorrect.

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