2.19 A solution containing 30 g of non-volatile solute exactly in of water has a vapour pressure of at . Further, of water is then added to the solution and the new vapour pressure becomes at . Calculate:
(i) molar mass of the solute
In this question we will find molar mass of solute by using Raoult's law .
Let the molar mass of solute is M.
Initially we have 30 g solute and 90 g water.
Moles of water :
By Raoult's law we have :-
or
or ------------------------------ (i)
Now we have added 18 g of water more, so the equation becomes:
Moles of H2O :
Putting this in above equation we obtain :-
or -----------------------------------(ii)
From equation (i) and (ii) we get
M = 23 u
So the molar mass of solute is 23 units.