What is the mass percent of a solution that contains 45.6 grams ethanol (#C_2H_6O#) in 254 grams of water?
The solution is 15.2% m/m.
The mass percent of a solution is simply the ratio between the mass of the solute, in your case ethanol, and the mass of the entire solution, which contains ethanol and water.
This is what's known as the percent concentration by mass, or %m/m.
So, the mass of the solution will be
This means that the mass percent of the solution is