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

Relative atomic masses are the larger numbers next to each element on the Periodic Table you are given.  (Remember to refer to the Key to check this.)  To get the relative formula mass, simply add the relative atomic masses taking into account the number of each atom present:

e.g. what it the relative formula mass of Mg(OH)2? Mg = 24, O = 16, H = 1

Mg × 1 plus O × 2 plus H × 2

24 + (2 × 16) + (2 × 1) = 58