Question

the vapour pressure of water is 12.3 KPa at 300 Kelvin calculate vapour pressure of 1 molal solution of a nonvolatile solute in it. ​

Answer 1

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mole fraction of solute ,

= 1/ 1+ 1000/18

= 0.0177

= p° - pA / p° = 0.0177

= 12.3 -pA /12.3 = 0.0177

pA = 12.08 kPa

Answer 2

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Vapor pressure also known as  equilibrium vapor pressure is defined as the pressure exerted by a vapor in the state of thermodynamic equilibrium with its condensed phases that may be solid or liquid,  at a given temperature in an enclosed system. .

What is 1 molal solution? :

A molality is defined as the  number of moles of solute dissolved in one kilogram of solvent.

Therefore, the formula weight for NaCl is 58, and 58 grams of NaCl dissolved in 1kg water would result in a 1 molal solution of NaCl. 1 molal solution means 1 mol of the solute is present in 1000 g of the solvent (water).Molar mass of water = 18 g mol - 1∴

Number of moles present in 1000 g of water = 1000/18= 55.56 molTherefore, mole fraction of the solute in the solution isx2 =  1 / (1+55.56) = 0.0177.

It is given that,Vapour pressure of water, p10 = 12.3 kPa.

Applying the relation, (P10 -  P1) / P10 = X2 ⇒ (12.3 - p1) / 12.3 =0.0177⇒ 12.3 - P1 = 0.2177⇒ p1 = 12.0823ie.

p1 = vapour pressure of 1 molal solution of a non-volatile solute= 12.08 kPa (approximately)

Hence, the vapour pressure of the solution is 12.08 kPa.

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