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

carbon dioxide molecule e.g. Find the percentage by mass for each element 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

We could carry out a similar calculation to workout the % by mass of oxygen, but there is no need. Since % add to 100 and we know that the % by mass for C is 27%, the the % by mass for oxygen is simply 100-27 = 73%. You could carry out a calculation similar to the one above just to check this value!

Example 2

Glucose has the formula C6H12O6. Calculate the % by mass of C,H and O in 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
 5 composition bt mass for glucose

Practice questions

Check your understanding - Questions on % composition by mass