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:

Worked examples

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

A_r of C=12   A_r of O=16

Start by calculating the M_r of carbon dioxide:
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 simply put in the values for the 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

Check your understanding - Additional questions on % composition by mass

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