 We have a two-component mixture of sodium hydrogen carbonate and sodium carbonate. We've masked out about 2.5 grams of the mixture. We're going to take that mixture and we're going to add it to an evaporating dish with a watch glass. We've masked the evaporating dish and watch glass previously. We add the solid to the evaporating dish. We cover it. We're now going to slowly add hydrochloric acid to the mixture. The hydrochloric acid will react with the sodium hydrogen carbonate and the sodium carbonate to produce water, sodium chloride, and carbon dioxide gas. The carbon dioxide gas will escape from the reaction mixture. The watch glass prevents any loss of compound from splattering. We'll continue to add the hydrochloric acid until all the compound has reacted. We'll know this by the fact that we will no longer see carbon dioxide being emitted. We'll gently swirl the mixture to make sure that all the compound has a chance to react. After it appears that all the compound has reacted, we'll rinse the bottom of the watch glass to make sure that any compound that may have splattered up on the bottom of the watch glass has a chance to react. We'll then evaporate the mixture to dryness. We've heated the sample to evaporate all the water and the excess HCl. What we're left with is the sodium chloride. We masked the evaporating dish and watch glass, and this allows us to calculate the mass of the sodium chloride produced. We use a mass of sodium chloride to back calculate how much sodium hydrogen carbonate and how much sodium carbonate were in the original sample.