1.16 calculate relative formula masses (Mr) from relative atomic masses (Ar)

The relative formula mass is just all the relative atomic masses of the atoms the compound contains. For example..

If you have MgCl2...

Relative atomic mass (Ar) of Mg = 24
Relative atomic mass (Ar) of Cl = 35.5

We need two times the Ar of Cl as there is two Cl atoms in the compound, therefore, the Relative formula mass of MgCl2 is...

24 + 35.5 + 35.5 = 95

and thats all there is :)

In an exam station, you can easily work this out as we are given the periodic table so we can look up the Ar of each atom and add them up.