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

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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.

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