How much $CaC{O}_3$ in mg must be present in 50ml of solution with 2.5ppm of Ca?

40g of Ca present in 100g of $CaC{O}_3$

${10}^{6}g$ of solution $\to 2.5g$of Ca

50ml $\to \frac{2.5}{{10}^6}{\text{x50=125x10}}^{-6}$

$=0.125mg$ of Ca.

40g Ca present in $\to 100g$$CaC{O}_3$

 0.125mg of Ca present in $\to ?$

$\begin{array}{l}\frac{100}{40}\text{x0.125mg}\\ \text{=0.3125mg}\end{array}$