# 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.

Answer:

(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:

⇒ …………………(i)

After the addition of 18 g of water:

Again, applying the relation

⇒ …………………..(ii)

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 .