# What does it mean to calculate hardness in terms of ppm of calcium carbonate? [closed]

PPM means per part million or milligram per litre of solution.Calculation of ppm for a given compound say $$x \pu{gms}$$ of $$\ce{Ca(HCO_3)}$$ in 1 liter of water would be done like

$$ppm= \frac{1000\cdot x}{1\pu{1L}}$$

But I do not understand by what's meant by in terms of $$\ce{CaCO3}$$ is it equivalents or weight? This was asked in an exam and the formula is not covered in my textbook.

It is meant as molar equivalent. $$\ce{CaCO3}$$ has the molar mass about $$\pu{100 g / mol}$$, $$\ce{Ca(HCO3)2}$$ has the molar mass cca $$\pu{ 162 g / mol}$$. Therefore $$\pu{100 g}$$ of the former is equivalent to $$\pu{ 162 g }$$ of the latter. The similar for the concentration expressed in ppm. E.g. if water contains $$\pu{1.62 ppm}$$ of $$\ce{Ca(HCO3)2}$$, it is equivalent to $$\pu{1 ppm}$$ of $$\ce{CaCO3}$$