Given:

$pH=7\mathrm{.}1$

Total Alkalinity $=203m\frac{g}{L}la{s}^{T}CaC{O}_3^{\text{'}\text{'}}$

Total Hardness $=297m\frac{g}{L^2}$ as $CaC{O}_3^{\text{'}\text{'}}$

Calcium Hardness $=235m\frac{g}{L^2}"$as $CaC{O}_3$

Since most hardness in natural waters is due to $C{a}^{2+}$ and $M{g}^{2+},$ you may assume:

Total Hardness $=C{a}^{2+}+M{g}^{2+}($each value expressed in $m\frac{g}{L}"$as $CaC{O}_3^3")$

Under this assumption, calculate the concentration of $M{g}^{2+}($in $m\frac{g}{L}"$as $CaC{O}^2")$ and enter the numerical value below.