A solution containing $30 g$ of a non-volatile solute in exactly $90 g$ water has a vapour pressure of $21.85 m m H g$ at $25^{\circ} C$ . Further 18 gm of water is then added to the solution. The resulting solution has a vapour pressure of 22.18 mmHg at $25^{\circ} C$ . Calculate (a) molar mass of the solute, and (b) vapour pressure of water at $25^{\circ} C$ .

(a) 30.6 g m o l^{- 1}, (b) 15.53 m m H g

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(a) 61.21 g m o l^{- 1}, (b) 23.99 m m H g

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(a) 65.45 g m o l^{- 1}, (b) 28.87 m m H g

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(a) 84.87 g m o l^{- 1}, (b) 39.98 m m H g

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Solution

The correct option is B $(a) 61.21 g m o l^{- 1}, (b) 23.99 m m H g$
$p_{1} = x_{1} p_{0}$
$21.85 m m H g = \frac{90 g / 18 g {m o l}^{- 1}}{(90 g / 18 g {m o l}^{- 1}) + (30 g / M)} \times p_{0}$
$22.18 m m H g = \frac{108 g / 18 g {m o l}^{- 1}}{(108 g / 18 g {m o l}^{- 1}) + (30 g / M)} \times p_{0}$

Dividing Eq. (2) by Eq. (1) we get.
$\frac{22.18}{21.85} = \frac{30 m o l + 180 g / M}{30 m o l + 150 g / M}$
$M = \frac{3933 g - 3327 g}{665.4 m o l - 655.5 m o l} = 61.21 g m o l^{- 1}$

Substituting this value of M in Eq. (1) we get.
$21.85 m m H g = \frac{5 m o l}{5 m o l + 30 g / 61.21 g m o l^{- 1}} \times p_{0}$
$p_{0} = 23.99 m m H g .$

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Similar questions

A solution containing 30 gms of non-volatile solute in exactly 90 gm water has a vapour pressure of 21.85 mm Hg at 25⁰C. Further 18 gms of water is then added to the solution. The resulting solution has a vapour pressure of 22.15 mm Hg at 25⁰C. Calculate the molecular weight of the solute.

Q. A solution containing

30 g

of a non-volatile solute in exactly

90 g

water has a vapour pressure of

21.85 m m

H g

25^{o} C

. Further

18 g

of water is then added to the solution.The resulting solution has a vapour pressure of

22.15 m m

H g

25^{o} C

. Calculate the molecular weight of the solute.

QUESTION 2.19

A solution containing 30 g of non-volatile solute exactly in 90 g of water has a vapour pressure of 2.8 kPa at 298 K. Further, 18 g of water is then added to the solution and the new vapour pressure becomes 2.9 k Pa at 298 K. Calculate (i) molar mass of the solute (ii) vapour pressure of water at 298 K.

Q. A solution containing