Question

Calculate the number of moles in 21.5g of calcium carbonate

Answer 1

n=m/M
where m is the mass in g
M is molar mass
M=100.0869g/moles but we round it to 100
n=21.5/100=0.215moles

Answer 2

First you calculate the molar mass of calcium carbonate :

CaCO3 = 40.078 + 12.0107 + 15.9994 x 3  => 100.0869 g/mol

therefore:

n =  mass of solute / molar mass

n = 21.5 / 100.0869 

n = 0.214 moles