How to Calculate Mass Percentage Compositions with Gusto

Mass percentage composition of a compound is a common issue in general chemistry courses in college. However, the principle is simple and just takes some practice to master.

When a chemistry question asks for the percentage composition of an element in a compound, you should assume that you are finding the mass percentage unless otherwise stated. Mass percentage is simply the mass of the element(s) in question, divided by the total mass of the compound multiplied by 100%.

If you are given the number of moles of the element, you can convert the moles into grams by multiplying by the molar mass. In some cases you might have to do this with the compound also to find the total mass. Once you have all relevant values in mass units (g, kg) you can do the procedure stated above to find the mass percentage.

As an example, assume that you have the compound formaldehyde CH2O, and the question asks for the percentage composition of oxygen O. That is, what percentage of the total mass is made up of oxygen? By looking up the molar masses of carbon, hydrogen and oxygen you find the molar mass of the compound to be 30 g/mol. The rest is easy. Oxygen has a molar mass of 16g/mol, so divide 16 by 30 and multiply by a 100% to get 53.3%.

If the question asked for the percent composition of carbon, you would follow the same procedure with the molar mass of carbon which is 12 g/mol, and you would get 40 %. If you in addition wanted to find the percentage composition of hydrogen, you have two options. Either you can find the molar mass of hydrogen to be 1g/mol, but since you have two moles of hydrogen present you multiply by two, divide by 30 and multiply by 100%. The second option would be to realize that all the elements will add up to 100%. Since you know the percentages of carbon and oxygen, the percent hydrogen must be 100% - (%O+%C). Either way it comes out to be 6.73% hydrogen.

In doing these problems it is vital that your answers make sense. Remember that since hydrogen is a very light weight gas, unless there is an abundance of hydrogen present the percent composition of hydrogen should be fairly small as compared to other heavier elements. You know that there is one mole each of carbon and oxygen, and since carbon is lighter than oxygen you expect the percent composition to also be smaller.