Determine the number of moles of oxygen in 3.20 moles of sodium hydrogen phosphate.

The chemical formula for sodium hydrogen phosphate is:

Na2HPO4

Looking at this we see that there are 4 oxygen atoms in each molecule. This means that for every mole of sodium hydrogen phosphate, there will be 4 moles of oxygen.

So simply take the moles of the molecule (3.20) and multiply it by the number of oxygen atoms in one molecule (4).

3.20 X 4 = 12.80 moles