Baking Soda and Acid: Is the Missing Mass Really Incomplete Reaction?
Students add 2.00 g of sodium bicarbonate (NaHCO3, molar mass 84.01 g/mol) to 25.0 mL of 1.00 M hydrochloric acid in an open Erlenmeyer flask. The mixture fizzes, and after five minutes the flask and contents have lost 0.72 g of mass. The students assume the mass loss is all CO2 from the reaction NaHCO3 + HCl → NaCl + H2O + CO2. One student calculates that the reaction should produce about 1.05 g of CO2 and concludes that the reaction stopped early. Another student argues that some CO2 stayed dissolved in the water, so the 1.05 g value cannot be compared directly with the balance reading. The teacher asks them to identify the limiting reactant, decide whether the data support incomplete reaction, and design a test to distinguish the two explanations.
1. Identify the limiting reactant and calculate the maximum mass of CO2 that could be released. Show your reasoning.
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n(NaHCO3) = 2.00 g / 84.01 g/mol = 0.0238 mol. n(HCl) = 0.0250 L × 1.00 mol/L = 0.0250 mol. The balanced equation uses a 1:1 ratio, so HCl is in slight excess (about 0.0012 mol) and NaHCO3 is limiting. Maximum CO2 = 0.0238 mol × 44.01 g/mol = 1.05 g. Therefore the measured 0.72 g is about 69% of the theoretical mass loss, not the full amount.
2. The measured mass loss is 0.72 g. Evaluate the claim that the reaction stopped early. What other explanation is plausible?
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An incomplete reaction is possible, but the data alone do not prove it. CO2 is soluble in water; in a five-minute open-flask experiment, some CO2 may remain dissolved or escape slowly, so the balance can under-report the reaction progress. Other possibilities are gas loss during addition, a balance or tare error, or not all bicarbonate reacting because acid was consumed locally. A test is to warm or swirl the flask for several minutes after the apparent stop and weigh again. If mass loss increases toward 1.05 g, dissolved CO2 was the issue. If it does not, test for unreacted acid or bicarbonate; adding a little more acid should produce more gas only if bicarbonate remains.
3. Describe a modified procedure that would give a more reliable yield, and explain how it addresses the problem.
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Use a closed system with a gas syringe or inverted buret to collect CO2 directly, and correct for water vapor pressure and temperature. Alternatively, heat the reaction mixture gently to drive off dissolved CO2, cool, and reweigh; run a blank with the same acid volume. These changes address the main weakness: the open-flask mass-loss method assumes all CO2 leaves and nothing else does. They also let the students compare the measured gas volume or mass with the limiting-reactant prediction.