A 5.00 gram mixture of CaO and CaCO3 is heated strongly. After heating, the mixture has a mass of 4.00 grams. What % of the original mixture is CaCO3?

Answer 1

The CaO is just an innocent bystander.
The loss in mass is due entirely to the amount of CaCO3 present initially. So convert loss of mass (grams CO2) to moles CO2 and convert that to moles CaCO3. Finally moles CaCO3 to grams CaCO3.
Then percent CaCO3 = (mass CaCO3/mass sample)*100 = ??