The vapour pressure of pure A is 10 torr and at the same temperature when 1g of B is dissolved in 20 g of A, its vapour pressure is reduced to 9.0 torr. If the molecular mass of A is 200 amu, then the molecular mass of B is

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90amu

100amu

75amu

120amu

answer is B.

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Detailed Solution

$\frac{P^{o} - P}{P^{o}} = X_{s} = \frac{n_{B}}{n_{A} + n_{B}} \frac{10 - 9}{10} = \frac{1 / M_{B}}{20 / 200 + 1 / M_{B}} On solving this, M_{B} = 90 amu$ .

Hence, the correct option (B). 90 amu .

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