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:

e.g. Find the percentage by mass for each element 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

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.
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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