What is the mole fraction of water in 200g of 95% (by mass) ethanol, c2h5oh?
c2h5oh= 46g/mol

For the given 200 g of 95% ethanol solution, ethanol is 190g and water is 10g. We determine the number of moles per component.
        ethanol:    (190 g ethanol) / (46 g/mole) = 4.13 moles
         water:      (10 g water)/(18 g/mole) = 0.55 moles

Total moles of the solution is 4.68 moles

Mole fraction of water:  = 0.55 moles/4.68 moles = 0.117