The Flask That Lost 1.62 Grams
Students add 5.00 g of baking soda to 50 mL of household vinegar (about 5% acetic acid by mass) in a 250 mL Erlenmeyer flask on a balance readable to 0.01 g. Three setups are run: Setup A is left open while the reaction fizzes; Setup B is immediately sealed with a rubber stopper; Setup C is sealed but the stopper is later found to be wet and not fully seated. Recorded mass changes are −1.62 g for A, −0.02 g for B, and −1.10 g for C. A student writes in the conclusion, "In Setup A, matter was destroyed, which is why the flask got lighter." The teacher knows the theoretical maximum carbon dioxide release is about 1.8 g given the acid as the limiting reactant, and must decide how to use the three numbers to build the conservation-of-mass idea rather than simply correcting the student.
1. Explain what each of the three mass changes indicates, and use the 1.8 g figure to comment on Setup A.
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Setup B is the clean case: the flask is closed, so no gas escapes, and the 0.02 g change is within the noise of a balance plus a warm flask and condensation. Matter is conserved. Setup A's −1.62 g is carbon dioxide leaving the open flask - the mass did not vanish, it moved into the room. Setup C is the instructive middle case: the mass loss sits between A and B because the loose stopper let gas escape, so the seal was partial. The 1.8 g theoretical maximum matters because 1.62 g is about 90% of it, which is consistent with a nearly complete reaction and a small amount of CO2 still dissolved in the solution or lingering in the flask headspace. If a group had reported −3.0 g, that would exceed the possible CO2 yield and would signal a spill, a mis-tare, or a recording error - a good check to teach. The judgment call is to make Setup B the reference and treat A and C as boundary cases, not as failures.
2. A student says Setup C "proves the law of conservation of mass is only true for closed containers." How should the teacher respond, and what would be a better statement of the law?
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The teacher should agree with the observation and reject the inference. The law was never violated in A or C; it applies to a defined system, and the student has simply chosen a system boundary that excludes the escaping gas. Re-draw the boundary to include the room's air and the accounting closes. A better statement is: "In a closed system, the total mass before and after a chemical change is the same; if measured mass changes, either the boundary is open or a measurement is in error." Then push the point with a concrete extension: if the class could collect the gas from Setup A and weigh it, the missing 1.62 g should reappear, and the mass of the dry residue should be less than 5.00 g plus the vinegar solids by exactly the escaping amount. Students who can relocate the system boundary, rather than memorize the slogan, are the ones who can later handle open-system combustion problems.
3. Propose a modification that would let the class measure the CO2 mass directly rather than inferring it from a difference, using only standard lab equipment.
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Run the reaction in a sealed flask connected by tubing to a gas syringe, or to an inverted water-filled graduated cylinder, and collect the evolved gas at room pressure. With acid limiting at about 0.042 mol, students should collect roughly 1.0 L of CO2 at room temperature and pressure if collection is complete; a syringe reading converted with the ideal gas law gives the same result. The direct measurement has two advantages: it converts an inference into an observation, and it exposes sources of loss that the difference method hides, such as gas staying dissolved in the acidic solution or expanding as the reaction warms. The teacher should still run Setup B as the conservation check, because the paired comparison - mass unchanged in a closed flask, gas measured in a collection system - gives students two independent routes to the same quantity and makes the stoichiometric prediction testable rather than decorative.