Higher and foundation tiers

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:

Example 1: Find the percentage by mass for each element present in a molecule of carbon dioxide (CO_{2}).

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 CO_{2}= 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.

Glucose has the formula C_{6}H_{12}O_{6}. 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