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  • 1gm of non-electrolyte solute disolved in 50gm of benzene lowered the freezing point of benzene by 0.4 K. Freezing point depression constant

1gm of non-electrolyte solute dissolved 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. The molar mass of solute is  

A. 252 g/mol

B. 256g/mol

C. 262 g/mol

D. 356 g/mol

Answers (1)

\\\text{As } \Delta \mathrm{T}_{\mathrm{f}}=\mathrm{K}_{\mathrm{f}} \times \mathrm{m} \\\\ \text{where, } \\ \mathrm{K}_{\mathrm{f}}=$ Freezing point depression constant; \\ $\Delta \mathrm{T}_{\mathrm{f}}=$ Depression in freezing point; \\ $\mathrm{m}=$ molality \\\\Now, $\Delta \mathrm{T}_{\mathrm{f}}=\mathrm{K}_{\mathrm{f}} \times \frac{\mathrm{W}_{2} \times 1000}{\mathrm{M}_{2} \times \mathrm{W}_{1}}$

\\\text{where, } \mathrm{W}_{2}=$ mass of solute; \\ $\mathrm{W}_{1}=$ mass of benzene i.e. solvent; \\ $\mathrm{M}_{2}=$ molar mass of solute \\ $\Rightarrow 0.40=5.12 \times \frac{1.0 \times 1000}{\mathrm{M}_{2} \times 50}$ \\$\Rightarrow \mathrm{M}_{2}=256$ \\ Thus, the molar mass of non-electrolyte is 256 .

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