Free residual chlorine concentration in water was measured to be $2 \mathrm{mg} / 1\left(\right.$ as $\left.\mathrm{Cl}_{2}\right)$. The pH of water is $8.5$. By using the chemical equation given below, the $HOCl$ concentration (in $\mu \text{moles} / \mathrm{l}$ ) in water is $\_\_\_\_\_\_\_\_\_$ (round off to one decimal place).
$$\mathrm{HOCl} \rightleftharpoons \mathrm{H}^{+}+\mathrm{OCl}^{-},\quad \mathrm{pK}=7.50$$
Atomic weight: $\mathrm{Cl}(35.5)$
