Question

What is percent composition by mass?

Answer 1

Percent composition by mass is the percentage by mass of each element in a compound.

Calculating percent composition by mass is a multi-step process. As an example, I'm going to use the molecule glucose (`"C"_6"H"_12"O"_6"`). I will demonstrate how to determine its molar mass, and then the percent composition by mass of each element in glucose.

1) Determine the molar mass of glucose (`"C"_6"H"_12"O"_6"`) by multiplying the number of atoms (subscripts) of each element times its molar mass (atomic mass in grams from the periodic table), and then add them together to get the molar mass of glucose..

C x 6 x 12.011g/mol = 72.066g/mol
H x 12 x 1.008g/mol = 12.096g/mol
O x 6 x 15.999g/mol = 95.994g/mol
Total = 180.161g/mol = molar mass of glucose

2) To calculate the percent composition by mass for each element in glucose, divide the molar mass of each element by the molar mass of glucose and multiply times 100. This will give the percent composition by mass for each element in the compound.

2a) Percent composition of C: `"72.066g/mol"/"180.161g/mol"` x 100 = 40.000%

2b) Percent composition of H: `"12.096g/mol"/"180.161g/mol"` x 100 = 6.7140%

2c) Percent composition of O: `"95.994g/mol"/"180.161g/mol"` x 100 = 53.282% .

Here is a video which discusses how to calculate percent composition from experimental data for a reaction of iron and oxygen which produces an iron oxide compound. Video from: Noel Pauller