(1) In order to prevent the calcium carbonate in the sample from failing to complete the reaction, the sample can be crushed to increase the contact area between the sample and hydrochloric acid, which can also accelerate the reaction rate;

Therefore, the answer is: increase the contact area and speed up the reaction rate;

(2) In the figure, each small scale on the mass axis represents 0.2g, so the total mass of the remaining substances after the gas is completely released is 73.8g；;

According to the law of conservation of mass, the mass of released gas carbon dioxide = 76g-73.8g = 2.2g.

Therefore, the answer is: 2.2g；;

(3) Let the mass of calcium carbonate in the limestone sample be x,

CaCO3+2HCl=CaCl2+H2O+CO2↑

100? forty-four

x 2.2g

10044＝x2.2g

Solution: x=5g.

The mass fraction of calcium carbonate in limestone sample = 5G6g×100% = 83.3%.

Answer: The mass fraction of calcium carbonate in this limestone sample is 83.3%.