Moles can be used to see how many molecules are in chemical compounds, too. Copper(II) chloride is an example. CuCl is its chemical formula. There is one copper atom (63.55) and two chlorine atoms (35.45 · 2 = 70.90). Add all the molar masses of the elements together to get the molar mass of the chemical compound (63.55 + 70.90 = 134.45). That means in 134.45 grams of copper(II) chloride, there is one mole of copper(II) chloride molecules. This concept is used to calculate how much chemicals are needed in a chemical reaction if no reactants (chemicals that are reacted) should be left. If too much reactant is used, there will be some reactants left in the chemical reaction.