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?

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 = ??

To find the percentage of CaCO3 in the original mixture, you need to calculate the mass of CaCO3 and then divide it by the initial mass of the mixture. Here's the step-by-step process:

1. Calculate the mass lost during heating:
Mass lost = Initial mass - Final mass
Mass lost = 5.00 g - 4.00 g
Mass lost = 1.00 g

2. The mass lost during heating represents the mass of CO2 released from the decomposition of CaCO3.

3. Since the molar mass of CO2 is 44.01 g/mol, you can find the number of moles of CO2 by dividing the mass lost by the molar mass:
Moles of CO2 = Mass lost / Molar mass of CO2
Moles of CO2 = 1.00 g / 44.01 g/mol
Moles of CO2 ≈ 0.022726 mol

4. Considering the balanced chemical equation for the decomposition of CaCO3 into CaO and CO2:
CaCO3 → CaO + CO2
The stoichiometric ratio between CaCO3 and CO2 is 1:1, meaning that 1 mole of CaCO3 decomposes to produce 1 mole of CO2.

5. Since the number of moles of CO2 is equal to the number of moles of CaCO3 initially present, the mass of CaCO3 can be calculated by multiplying the moles of CO2 by the molar mass of CaCO3:
Mass of CaCO3 = Moles of CO2 x Molar mass of CaCO3
Mass of CaCO3 ≈ 0.022726 mol x 100.09 g/mol (molar mass of CaCO3)
Mass of CaCO3 ≈ 2.273 g

6. Finally, calculate the percentage of CaCO3 in the original mixture by dividing the mass of CaCO3 by the initial mass of the mixture and multiplying by 100:
Percentage of CaCO3 = (Mass of CaCO3 / Initial mass of mixture) x 100
Percentage of CaCO3 = (2.273 g / 5.00 g) x 100
Percentage of CaCO3 ≈ 45.5%

Therefore, approximately 45.5% of the original mixture is CaCO3.