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:

Worked examples

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

A_r of C=12   A_r of O=16
M_r of carbon dioxide = (A_r of carbon) + (2xA_r of oxygen)
= 12 + 32
=44

M_r of CO₂= 44.
Now using the formula above we put in the values of A_r and M_r

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.
A_r of C=12 A_r of O=16 A_r of H=1
M_r of glucose = (A_r of carbon x6) + (A_r of hydrogen x12) + (A_r of carbon x6)
= 72 + 12 + 96
=180

Check your understanding - Questions on % composition by mass

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