Excess amount of solid calcium sulphate $\left(\mathrm{CaSO}_4\right)$ was added to a pure water sample $(\mathrm{pH}=7)$ so that some solids remain undissolved at the equilibrium. The solubility product of $\mathrm{CaSO}_4$ is $2 \times 10^{-5}$ $\mathrm{mol}^2 / \mathrm{L}^2$. The molar concentration of $\mathrm{SO}_4^{2-}$ in this water sample at equilibrium will be ____________ $\mathrm{mol} / \mathrm{L}$ $\text{(rounded off to three decimal places)}$.
