What does "in excess" mean? "In excess" simply means you have added more reagent than will react with a particular substance. In chemical analysis, it is important to ensure all of the substance you are analyzing reacts. In this problem, excess hydrochloric acid was added. If it had not been added in excess, there would be a possibility that not all of the calcium carbonate had been converted to carbon dioxide. If this was the case, the amount of carbon dioxide produced would not be a measure of how much calcium carbonate was initially present. You may wonder how you can know that the hydrochloric acid is in excess since we don"t know how much calcium carbonate is present. An educated guess can usually be made; you usually have a ballpark idea of how much is present. If not, you can assume that all of the antacid tablet is calcium carbonate and then calculate how much acid would be needed to react completely with 1.00 g CaCO₃.