[lrisNil]

A solid sample initially contains a mixture of CaC2O4 and CaCO3.
CaC2O4 decomposes around 500 °C and CaCO3 decomposes around 700 °C

This TGA graph plots the mass of solid remaining as the sample temperature is raised range 350 to 800 °C .

What is the mass percent of calcium carbonate in the original mixture?

First I wrote the chemical equations:
CaC2O4 ----> CaCO3 + CO
CaCO3 ----> CaO + CO2

And I see that the first drop from 9.1mg to 7.8mg is about 1.3mg drop.
So that means the CO escaped and there can only be CaCO3 left I think.
So then I did 7.8/9.1 *100 but this answer is incorrect.

Any help appreciated, I'm super confused.

[Borek]

CaCO₃ produced during the decomposition doesn't count as CaCO₃ originally present in the sample.

If there were 1.3 mg of CO produced, how many mg of oxalate decomposed? (Assuming your decomposition reaction is a correct one, no idea if it is, even if it looks plausible.)