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.

16. When the TGA began, approximately what percent of the mass of the sample was made up of H2O ?

Answer and Explanation

Your Answer is

Correct Answer is F

Explanation

Method 1: A smart but common method for scumbags, reading pictures. The initial CaC2O4•H2O is 100 g, after decomposing and losing water, CaC2O< sub>4, the mass becomes about 88 g, and the mass of lost H2O is about 100-88=12 g, so the proportion of water is about 12/100

Method 2: Calculating the molecular mass in a stupid but learned way. The molecular mass of CaC2O4•H2O is 146, and the molecular mass of H2O is 18 , 18/146≈12%