The concentration of ${\mathrm{S}}^{2-}$ in a $0.1{\mathrm{MH}}_2\mathrm{S}\left(K_{\mathrm{a}1}{K}_{\mathrm{a}2}=1.1\times {10}^{-21}{\mathrm{M}}^2\right)$ which is also $0.1 \mathrm{M}$ in ${\mathrm{H}}^{+}$ is

$\left[{\mathrm{S}}^{2-}\right]=\frac{K_{\mathrm{al}}{K}_{\mathrm{a}2}}{{\left[{\mathrm{H}}^{+}\right]}^2}\left[{\mathrm{H}}_2\mathrm{S}\right]=\frac{\left(1.1\times {10}^{-21} {\mathrm{M}}^3\right)(0.1 \mathrm{M})}{[0.1 \mathrm{M}{]}^2}=1.1\times {10}^{-20} \mathrm{M}$

The quantity of solute molecules present in one kilogram me of the solvent is known as the molality. For instance, a 1 molal NaOH solution is one that contains 1 mole of NaOH in 1 kg of water.