Find the mass percentage for a solution prepared from 15 grams of ethanol and a 100 millilitres of acetone. The first thing I want to do is write down my definition of a mass percentage. So that I know what I'm looking for. So I say that I have the mass of my solute, and that's going to be in units of grams over the mass of my solution, and that will be in grams times 100. And I need to remember that the solution includes both the solute, and the solvent. So I see that ethanol is present in the smaller amount, so that's going to be my solute and that acetone is present in the larger amount, that's going to be my solvent. So now, I can actually figure out my mass of my solvent because, I already know the mass of the solute, but I'm going to need to know the mass of the solvent. So I see that I have 100 milliliters of acetone, and I'm going to use the given density. And I'm actually going to multiply that, because that puts the milliliters on the bottom. And what that allows to happen is the cancellation of those milliliters. So the units I'm left with now is the units of grams. Which is what I'm looking for. So, 79.1 grams of the acetone, that's my solvent. Now, I can take my gram amount, and plug into my equation. I know that the mass of my solvent is, or solute, is 15.0 g, and I'm going to divide that by. The 79.1 grams for my solvent plus the 15.0 grams from my solute. Then I'm going to multiply that by 100. And when I do, I get 16.0%. And because this is a percent by mass. I'm going to show it as m over m, indicating mass percent