How do I calculate the relative formula mass of Calcium carbonate (CaCO3)

The relative formula madd of a compound comes from the total mass of all of the elments within the compound.

So in the case of calcium carbonate (CaCO₃) we have;

                              1 x Ca

                               1 x C

                               3 x O

So we first need to know the relative mass nnumber (Mr) of these individual elements. This information comes from the periodic table and is determined by the number of protons + neutrons in an atom.

            Mass number = protons + neutrons

So if we look at the elements in our compound we have;

                                     ²⁰Ca₄₀  ⁶C₁₂  ⁸O₁₆

In each case the mass number is the bottom right number (in bold), we can now calculate our relative formula mass using these numbers so we have;

           Ca(1 x 40) + C(1 x 12) + O(3 x 16)

    =     40 + 12 + 48

   =       100 g/mol

The units of relative formula mass are g/mol, which tells us that 1 mole of CaCO₃ has a mass of 100 g.