Assuming that $\mathrm{Ba}(\mathrm{OH})_2$ is completely ionised in aqueous solution under the given conditions the concentration of $\mathrm{H}_3 \mathrm{O}^{+}$ions in 0.005 M aqueous solution of $\mathrm{Ba}(\mathrm{OH})_2$ at 298 K is $\_\_\_\_$ $\times 10^{-12} \mathrm{~mol} \mathrm{~L}^{-1}$. (Nearest integer)