Air contains 21% oxygen by volume. Calculate the volume of air, in dm3\text{dm}^3dm3, needed to provide enough oxygen to react completely with 4.20 dm3 of hydrogen gas. The balanced chemical equation for the reaction is:
2H2(g)+O2(g)→2H2O(g) 2\text{H}_2\text{(g)} + \text{O}_2\text{(g)} \rightarrow 2\text{H}_2\text{O(g)} 2H2(g)+O2(g)→2H2O(g)