During an experiment, hydrogen $\left({\mathrm{H}}_2\right)$ and oxygen $\left({\mathrm{O}}_2\right)$ gases are reacted in an electric arc to produce water as follows: $2\mathrm{H}+{\mathrm{O}}_2\stackrel{\mathrm{E}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{t}\mathrm{y}}{\to }2{\mathrm{H}}_2\mathrm{O}$
The experiment is repeated three times and data tabulated as shown below:

Experiment number	Mass of ${\mathrm{H}}_2$ reacted	Mass of ${\mathrm{O}}_2$ reacted	Mass of ${\mathrm{H}}_2\mathrm{O}$ produced
$1$	$2\mathrm{g}$	$16\mathrm{}\mathrm{g}$	$18\mathrm{}\mathrm{g}$
$2$	$4\mathrm{g}$	$32\mathrm{}\mathrm{g}$	$36\mathrm{}\mathrm{g}$
$3$	$-$	$-$	$9\mathrm{g}$

During $3\mathrm{r}\mathrm{d}$ experiment the researcher forgot to list masses of ${\mathrm{H}}_2$ and ${\mathrm{O}}_2$ used. So, if the law of constant proportion is correct then find mass of ${\mathrm{O}}_2$ used during $3\mathrm{r}\mathrm{d}$ experiment.