25 g of ethyl alcohol is dissolved in 100 ml of water (density = 0.3 at 20 oc). what is the % w/w for the ethyl alcohol in the resulting solution?

Percentage by mass is the amount in mass of a component in a mixture per 100 unit of mass of the total mixture. Percentage by mass is the same as  %w/w. We can determine this by dividing the mass of the solute with the total mass of the mixture. However, from the problem statement, we are given the volume of the water so there is a need to convert this value to mass by using the density of water. We calculate as follows:

Mass of solution = 100 mL (0.99993 g/mL) water + 25 g EtOH
Mass of solution = 124.993 g solution

%w/w = 25 g / 124.993 g x100
%w/w = 20% of EtOH