Higher and foundation tier

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

percentage by mass equation

Worked examples

E.g. Find the percentage by mass for each element present in carbon dioxide.

Ar of C=12   Ar of O=16
Mr of carbon dioxide = (Ar of carbon) + (2xAr of oxygen)
= 12 + 32

Mr of CO2= 44.
Now using the formula above we put in the values of 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