If 30 ml of ${\mathrm{H}}_2$ and 20 ml of ${\mathrm{O}}_2$ react to form water, what is left at the end of the reaction?

For the given reaction,

${\mathrm{H}}_2\left(\mathrm{g}\right) + \frac{1}{2}{\mathrm{O}}_2\left(\mathrm{g}\right) \to {\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)$

$\begin{array}{ccc}30 & 20 & 0\end{array}$

$0 (20-15) 30$

at the end of the reaction, 30 ml of water and 5 ml of ${\mathrm{O}}_2$ and 10 ml of ${\mathrm{H}}_2$ will be left.