Relative Formula Mass

Think of relative formula mass as the molecular weight of any compound - it's basically how heavy a molecule is compared to a standard carbon-12 atom. You'll find this concept everywhere in chemistry, from balancing equations to working out how much product you'll get in reactions.

The process is actually dead simple once you get the hang of it. First, you count up all the atoms of each element in the chemical formula. Then you add together all their relative atomic masses (Ar values) which you can find on any periodic table.

Let's try carbon dioxide (CO₂) as an example. You've got one carbon atom $Ar = 12$ and two oxygen atoms $Ar = 16 each$. So the Mr (that's the symbol for relative formula mass) equals 12 + 16 + 16 = 44.

Top Tip: Even ionic compounds like sodium chloride follow exactly the same rules, even though they don't actually exist as separate molecules!