The answer lies in the stoichiometry of the reaction. If u look at the number BEFORE the reagent u will see the ratios of the reagents.
Theodore Richards; first scienstist to recieve nobel prize in Chemistry, earning the award "in recognition of his exact determinations of atomic weights of a large number of chemical elements."
Ca=40
C=12
O=16
1 mole of CaCO3 has 100 grams
So 50 grams is 0.5 mole