Alkalinity of water, in equivalent/litre (eq/litre), is given by

$${\{HC{O_3}^-}\} + 2{\{C{O_3}^{2-}}\} + {\{OH^-\}}\ –\ {\{H^+\}}$$

where, $\{\ \ \}$  represents concentration in $\text{mol/litre}$. For a water sample, the concentrations of $HC{O_3}^- = 2 \times 10^{-3}\ \text{mol/litre},\: C{O_3}^{2-} = 3.04 \times 10^{-4}\ \text{mol/litre}$ and the $pH$ of water $= 9.0.$ The atomic weights are$: Ca = 40;\:C = 12;$ and $O = 16.$ If the concentration of $OH^-$ and $H^+$ are NEGLECTED, the alkalinity of the water sample $\text{(in mg/litre as}\: CaCO_3),$ is

1. $130.4$
2. $100.0$
3. $50.0$
4. $65.2$