# 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.

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