How much oxygen will react with 4 moles of hydrogen?

We write the reaction:

2H2 + O2 = 2H2O (water is formed)

The reaction is written in order to make it clear what the odds are. They must be taken into account in solving any problems.

Above hydrogen (H) we write “4 mol” (as given), and below – “2 mol” (because of the coefficient next to hydrogen). Near oxygen (O) we write “1 mole” from below (because there is no coefficient near it). We get the proportion:

4 mol – 1 mol

2 mol – x

Find x:

x = 2 * 1/4 = 0.5 mol

Answer: 0.5 mol.