Higher and foundation tiers

Percentage composition by mass

Once you have found the relative formula mass it is possible to calculate the percentage by mass for each element in a compound. Simply use the formula shown below:

percentage by mass equation

Worked examples

Example 1: Find the percentage by mass for each element present in a molecule of carbon dioxide (CO2).

Ar of C=12   Ar of O=16

Start by calculating the Mr of carbon dioxide:
Mr of carbon dioxide = (Ar of carbon) + (2xAr of oxygen)
= 12 + 32

Mr of CO2= 44.
Now using the formula above simply put in the values for the Ar and Mr:

carbon dioxide

Just check once you have done all your calculations that the total % composition for all the elements in the compounds adds up to 100%. If it does not then you have made an arithmetic error in your calculation.

Example 2

Glucose has the formula C6H12O6. Calculate the % by mass of C, H and O in glucose. Use the same method as above to find the % composition by mass for each element in a molecule of glucose.
Ar of C=12   Ar of O=16   Ar of H=1
Mr of glucose = (Ar of carbon x6) + (Ar of hydrogen x12) + (Ar of carbon x6)
= 72 + 12 + 96

 % composition by mass for glucose

Practice questions

Check your understanding - Questions on % composition by mass

Check your understanding - Additional questions on % composition by mass