In order to determine the mass fraction of calcium carbonate in a batch of limestone samples,

Solution: (1) After the fifth addition of hydrochloric acid, the quality of the remaining samples remained unchanged, indicating that the calcium carbonate reaction was complete after the fourth addition of hydrochloric acid.

(2) It is known from the second time that every 5g of hydrochloric acid can react with 0.5g of calcium carbonate, and the total mass of the sample is 2.0g. If the unreacted mass is 0.2g from the fifth time, the mass fraction of calcium carbonate is 1.8/2.0=90%.

③CaCO3+2 HCl = = CaCl 2+CO2 =+H2O。

Reaction ratio (%) 100:: 73

0.5:x

X = 0.365g is obtained.

0.365/0.5=73%