Hello, here goes the problem: The mass of calcium carbonate upon thermal decomposition decreased by 1/5. (a) How many molecules of CaCO3 per 100 molecules were decomposed to CaO and CO2. (b) The content of CaO in the final sample express in molar fraction. My solution: I set the starting sample to 100g which corresponds to 1 mole of CaCO3. The decrease of mass by 1/5 corresponds to 20g or 0.2 mole. So the solution seems to me to be 20 molecules per 100 molecules. Ad (b) If 0.2 mole of CaCO3 were decomposed, 0.2 mole of CaO and 0.2 mole CO2 should be formed. Thus the molar fraction of CaO in final samples should be 0.2/(0.2+0.2+0.8) = 1/6. Yet the provided solutions from two scources differs. Where, if so, did I make mistake? Thank you for your help.