If the concentration of Ca is 56.4 mg/L and of Mg is 8.8 mg/L in water, what is the total hardness of the water expressed in mg/L of CaCO3?

You use the atomic number to find out how many protons and neutrons there are in the atom. you use the atomic mass to find out how many protons and electrons there are in the atom. 

Ais your answer to this

Molarity is moles of solute over liters of solution. idk if that is what you are talking about, but you can convert moles of solute to grams of solute using dimensional analyzes by using molar mass found on the periodic table.