Passage III
As calcium oxalate hydrate (CaC2O4·H2O) is heated, it first dehydrates to yield calcium oxalate (CaC2O4). Next, the CaC2O4 decomposes into calcium carbonate (CaCO3) and carbon monoxide (CO). The CaCO3 then further decomposes into CaO and CO2 (see equations below).
CaC2O4·H2O(solid, s) 
CaC2O4(s) + H2O(gas, g)
CaC2O4(s)CaCO3(s) + CO(g)
CaCO3(s)CaO(s) + CO2(g)
In a thermogravimetric analysis (TGA), the mass of a sample is monitored as the sample's temperature is steadily increased. A nonreactive gas is streamed over the sample to carry away any gaseous products. The figure shows the results of a TGA of a 100 g sample of CaC2O4·H2O.
13. If the sample of CaC2O4·H2O had been 50 g, the mass of the sample at 900°C would have been approximately:
Your Answer is
Correct Answer is A
Explanation
Corresponding to the above figure, when CaC2O4•H2O is 100 g, and the abscissa temperature is 900, the reaction product The mass of CaO is about 38 g;
So when we have 50 g CaC2O4•H2O , the mass of CaO should be half of that on the picture, that is, 38/2=19 g, which is the closest to option A 20 g