Question -
Answer -
Further, 18 g of water is then added to
the solution and the new vapour pressure becomes 2.9 kPa at 298 K. 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 M g mol−1
Now,the no. of moles of solvent (water), 
And, the no. of moles of solute,
Applying the relation:
After the addition of 18 g ofwater:
Again, applying the relation:
Dividing equation (i) by (ii),we have:
Therefore, the molar mass of thesolute is 23 g mol−1.
(ii) Putting the value of ‘M’ in equation (i),we have:
Hence, the vapour pressure ofwater at 298 K is 3.53 kPa.