A mixture of calcium oxide.CaO, and calcium carbonate, CaCO3, that had a mass of 1.844 g was heated until all the calcium carbonate was decomposed according to the following equation. After heating the sample weighed 1.462g. Calculate the masses of CaO and CaCO3 present in the original sample.
          CaCO3 ---> CaO + CO2

CaCO3 ---> CaO + CO2
here you work in reverse way
convert 1.462g(this is due to CaO only) into moles of CaO
Convert moles of CaO obtained above in moles of CaCO3 using balanced equation
finally convert moles of caCO3 into mass of CaCO3
mass of CaCO3  X 100%  / mass of sample  is the %purity of the sample