Consider a discrete random variable $X$ whose probabilities are given below. The standard deviation of the random variable is _________ (round off to one decimal place).

$$ \begin{array}{|c|c|c|c|c|} \hline x_1 & 1 & 2 & 3 & 4 \\ \hline P\left(X=x_i\right) & 0.3 & 0.1 & 0.3 & 0.3 \\ \hline \end{array} $$

The bacteria mainly responsible for crown corrosion in a sewer is $\qquad$

Free residual chlorine concentration in water was measured to be $2 \mathrm{mg} / \mathrm{l}$ (as $\mathrm{Cl}_2$ ). The pH of water is 8.5. By using the chemical equation given below, the HOCl concentration (in $\mu$ moles $/ \mathrm{l}$ ) in water is $\qquad$ (round off to one decimal place).

$$ \mathrm{HOCl} \rightleftharpoons \mathrm{H}^{+}+\mathrm{OCl}^{-}, \mathrm{pK}=7.50 $$

Atomic weight: Cl(35.5)