I need help with this problem... I seem to be having troublefinding the mole fra

Question

I need help with this problem... I seem to be having troublefinding the mole fraction part of the equation... I have thecorrect answer and correct way to solve it, but there is a questionin the process of getting the answer.
any help would be much appreciated!

Q:
The vapor pressure of pure water at 35degrees is 42.2mmHg. What isthe vapor pressure of a mixture of 15g sucrose(C12H22O11, molar mass of342.3g/mol) and 85g water?
The equation used to solve this problem is:
Psolvent= XsolventP°solvent

I got the answer to be 41.8 mmHg by solving this equation

mole fraction (Xsolvent): 4.7mols water/ (.044molssucrose + 4.7 mols water) = 0.9907
then multiply0.9907 x 42.2mmHg = to get 41.8mmHg

My question is, why do you find the mole fraction of only water andnot of sucrose.
As in why don't we equate the mole fraction to be: 0.044molssucrose/(0.044 + 4.7 mols water) ?

Explanation / Answer

The equation used is nothing but Raoult's law.

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