The mass of CO2 evolved represents the change in mass before and after the reaction (i.e., when the acid was transferred from the pipet to the carbonate sample). For the table of data above, the mass of carbon dioxide evolved represents the difference between the (mass of beaker, tube, HCl, sample before reaction) and the (mass of beaker, tube, HCl, sample after reaction).

   
Mass of CO2 evolved Trial 1 = (10.2758 g - 10.0536 g) = 0.2222 g CO2 evolved