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:

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 CO_{2}= 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 C_{6}H_{12}O_{6}. 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