**Find the mass of urea (CH4N2O) needed to prepare 51.8g of a solution in water in which the mole fraction of urea is 7.55×10−2.**

Is this a valid assumption?

The molar mass of the solution is MM(H2O) + MM(NH4N2O)

So then, MM(sol'n) = (18.02 + 60.062)g/mol sol'n = 78.082g/mol sol'n

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If the mole fraction of urea is 0.0755, then the mole fraction is water is 0.9245.

I would like a hint, and know if my assumption will work.