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Addition Reaction

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Question

On dissolving 0.5 g of a non-volatile non-ionic solute in 39 g of benzene, its vapour pressure decreases from 650 mm of Hg to 640 mm of Hg. The depression in freezing point of benzene (in K) upon addition of the solute is: ( Given: The molar mass and the molal freezing point depression constant for benzene are $78 g m o l^{- 1}$ and $5.12 K K g m o l^{- 1}$ , respectively

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Solution

$\frac{p_{0} - p_{s}}{p_{s}} = i (\frac{n_{s o l u t e}}{n_{s o l v e n t}}) \frac{650 - 640}{640} = 1 \times \frac{0.5 \times 78}{M \times 39} \Rightarrow M_{s o l u t e} = 64 g Δ T_{f} = K_{f} \times m o l a l i t y = 5.12 \times \frac{0.5 \times 1000}{64 \times 39} Δ T_{f} = 1.02 K$

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