Science, Tech, Math › Science Percent Composition by Mass Example Worked Chemistry Problems Share Flipboard Email Print André Saß / EyeEm / Getty Images Science Chemistry Basics Chemical Laws Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Todd Helmenstine Todd Helmenstine is a science writer and illustrator who has taught physics and math at the college level. He holds bachelor's degrees in both physics and mathematics. our editorial process Todd Helmenstine Updated December 10, 2019 Percent composition by mass is a statement of the percent mass of each element in a chemical compound or the percent mass of components of a solution or alloy. This worked example chemistry problem works through the steps to calculate percent composition by mass. The example is for a sugar cube dissolved in a cup of water. Percent Composition by Mass Question A 4 g sugar cube (Sucrose: C12H22O11) is dissolved in a 350 ml teacup of 80 °C water. What is the percent composition by mass of the sugar solution? Given: Density of water at 80 °C = 0.975 g/ml Percent Composition Definition Percent Composition by Mass is the mass of the solute divided by the mass of the solution (mass of the solute plus mass of the solvent), multiplied by 100. How to Solve the Problem Step 1 - Determine mass of solute We were given the mass of the solute in the problem. The solute is the sugar cube. masssolute = 4 g of C12H22O11 Step 2 - Determine mass of solvent The solvent is the 80 °C water. Use the density of the water to find the mass. density = mass/volume mass = density x volume mass = 0.975 g/ml x 350 ml masssolvent = 341.25 g Step 3 - Determine the total mass of the solution msolution = msolute + msolvent msolution = 4 g + 341.25 g msolution = 345.25 g Step 4 - Determine percent composition by mass of the sugar solution. percent composition = (msolute / msolution) x 100 percent composition = ( 4 g / 345.25 g) x 100 percent composition = ( 0.0116) x 100 percent composition = 1.16% Answer: The percent composition by mass of the sugar solution is 1.16% Tips for Success It's important to remember you use the total mass of the solution and not just the mass of the solvent. For dilute solutions, this doesn't make a huge difference, but for concentrated solutions, you'll get a wrong answer.If you're given the mass of solute and mass of solvent, life is easy, but if you're working with volumes, you'll need to use density to find the mass. Remember density varies according to temperature. It's unlikely you'll find a density value corresponding to your exact temperature, so expect this calculation to introduce a small amount of error into your calculation.