Solve this mole fraction calculation with incomplete information
I don't think you can solve that unless you know additional information like mass of water added or volume of water added, or the density of the final solution.

You could approximate by dividing mass of ethanol by density of ethanol to get volume of ethanol, and saying volume of water added is 50mL-(volume ethanol), but this is only an approximation because volumes are not additive when the two solutions are different.

The error could be estimated from this graph:



source: http://en.wikipedia.org/wiki/Partial_molar_property

or the data from the graph could be used to improve the accuracy of the answer

or use this density table: http://www.handymath.com/cgi-bin/ethanolwater3.cgi?submit=Entry