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Depression in Freezing Point

Calculate the...

Question

Calculate the mass of compound (molar mass = $256 g {m o l}^{- 1}$ ) to be dissolved in $75 g$ of benzene to lower its freezing point by $0.48 K$ ( $K_{f} = 5.12 K k g {m o l}^{- 1}$ ).

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Solution

Given data is

$T_{f} = 0.48 K, K_{f} = 5.12 k g m o l^{- 1}$

$M_{2} = 256 g m o l^{- 1}, W_{1} = 75 g, W_{2} = ?$

We know that

$Δ T_{f} = \frac{K_{f} \times w_{2} \times 1000}{M_{2} \times w_{1}}$

$w_{2} = \frac{Δ T_{f} \times M_{2} \times w_{1}}{K_{f} \times 1000}$

$\Rightarrow \frac{0.48 \times 256 \times 75}{5.12 \times 1000}$

$\Rightarrow 18 g$

This the required solution.

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