Calculate the number of moles of caco3 (calcium carbonate, or limestone) in a 20.0g sample of this substance

First calculate for the molar mass of the given formula unit, CaCO₃. This can be done by adding up the product when the number of atom is multiplied to its individual molar mass as shown below.

     molar mass of CaCO₃ = (1 mol Ca)(40 g Ca/mol Ca) + (1 mol C)(12 g of C/1 mol of C) + (3 mols of O)(16 g O/1 mol O) = 100 g/mol of CaCO₃

Then, divide the given amount of substance by the calculated molar mass.
            number of moles = (20 g)(1 mol of CaCO₃/100 g)
             number of moles = 0.2 moles of CaCO₃

0.2 moles