(1) The chemical equation for producing carbon dioxide in the laboratory is CaCO3+2HCl═CaCl2+H2O+CO2↑;
(2) The average relative molecular mass of air is 29. If the relative molecular mass of a gas is greater than 29, the density of the gas is greater than that of air; if it is less than 29, the density is less than that of air. The relative molecular mass of carbon dioxide is 12+16×2=44, so the density of carbon dioxide is greater than that of air. The upward air exhaust method can be used to collect carbon dioxide. If there are 50g of limestone with a mass fraction of 80% in an Erlenmeyer flask, the mass of calcium carbonate is: 50g × 80% = 40g, assuming that up to x carbon dioxide gas can be produced. CaCO3+2HCl═CaCl2+H2O+CO2↑;
100?44
40gx
10040g＝44x x=17.6g
Therefore, the answer is: (1) CaCO3+2HCl═CaCl2+H2O+CO2↑;
(2) 44, upward air exhaust method. 17.6g．