Calculate the number of moles in 20g CaCO3?

Molar mass of CaCO₃ = (40 + 12 + 16) g/mol = 100 g/mol

No. of moles of CaCO₃ = (20 g) / (100 g/mol) = 0.2 mol

First calculate the 'Mr' ( Relative Molecular Mass).
To do this you need a copy of the Periodic Table.
In the formula there is
1 x Ca = 1 x 40(atomic mass) = 40
1 x C = 1 x 12 = 12
3 x O = 3 x a6 = 48
40 + 12 + 48 = 100 ( The Mr of CaCO3) .

Next using the equation
moles = mass(g) / Mr
mol(CaCO3) = 20 / 100 = 1/5 = 0.2
Hence there are 0.2 moles of CaCO3 present .