Chemistry Class 12 NCERT Solutions: Chapter 2 Solutions Part 10

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Q: 19. A solution containing of non-volatile solute exactly in of water has a vapour pressure of at 298 K. Further, of water is then added to the solution and the new vapour pressure becomes at . Calculate:

i. Molar mass of the solute

ii. Vapour pressure of water at 298 K.


(i) Let, the molar mass of the solute be

Now, the no. of moles of solvent (water),

And, the no. of moles of solute,

Applying the relation:


After the addition of 18 g of water:

Again, applying the relation


Dividing equation (i)by (ii), we have:

Therefore, the molar mass of the solute is .

(ii) Putting the value of 'M' in equation (i), we have:

Hence, the vapour pressure of water at .