Compounds have a relative formula mass, M

• If you have a compound like MgCl2 then it has a relative formula mass, M, which is just the relative atomic masses of all the atoms in the molecular formula added together

• Calculating relative formula mass is straight forward enough, but things can get a bit more confusing when you start working the percentage compositions of compounds

You can calculate the % mass of an element in a compounds

• Percentage mass of an element in a compounds = Ar x number of atoms of that element / Mr of the compounds x 100

• These questions may involve recurring decimals but you will always have a calculator so you should be able to work it out

Calculating relative formula mass

• The relative formula mass of a substance made up of molecules is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.

• Relative formula mass has the symbol, Mr. To calculate the Mr for a substance:

1. work out how many atoms of each element there are in the chemical formula

2. add together the Ar values for all the atoms of each element present