By Ann Levinson

About the Authors

Ann Levinson currently teaches high school chemistry at Chicagoland Jewish High School in Deerfield, Illinois. Previously Ann was a chemistry teacher at Niles West High School in Skokie, Illinois. She has a wealth of experience, as she has taught many different levels of chemistry for over forty years. In addition to teaching she also serves as a College Board Consultant for AP Chemistry.

Felix Rosen is currently a sophomore student, enrolled in Honors Chemistry at Chicagoland Jewish High School. 

To teach stoichiometry, I have students heat various masses of sodium bicarbonate in evaporating dishes on hot plates and compare molar ratios of the sodium bicarbonate to the sodium carbonate. The 2:1 ratio works for class data very nicely with relatively no error.

2NaHCO₃(s) ↔Na₂CO₃(s) + CO₂(g) + H₂O (g)

Next, students complete a single displacement reaction

Cu(s) + 2 AgNO₃ (aq) ↔ Cu(NO₃)₂ (aq) + 2Ag(s)

using a large excess of copper and again they compare molar ratios. This data rarely comes out as perfect as in the decomposition lab, but the students find the experiment very exciting because of the blue colored copper(II) nitrate solution and the fact that they “make” silver metal. We talk about possible sources of error, including copper wire that is included in the silver mass, some silver adhering to the copper wire, or a possible side reaction of the copper wire such as forming red Cu₂O.

One student suggested an alternative procedure. He proposed that if stoichiometric amounts of both reactants were used, that might eliminate some error. I allowed him the opportunity to test out his theory. He did two trials, and below are pictures of the original experiment with excess copper solid (Figure 1) and his modified experiment with 2:1 molar ratios (Figure 2), and also a summary of his lab.

Lab Summary: Reducing error

With the current emphasis on inquiry, when a student suggests an alternative method for an experiment, I offer them the chance to try their idea. This student came during his lunch on three separate days and conducted the experiment with the exact stoichiometric amounts rather than having any excess copper. He mentioned that by doing this, he solidified his understanding of how to do the calculations. He effectively eliminated any chance of excess copper wire being included in the mass of the silver as well as having some silver metal adhere to excess copper wire. In addition, there did not appear to be any side reactions with the copper since, there was no excess that could react. I encouraged him to then read scientific journals to get an idea of how to present his findings.

I think it’s very worthwhile, if time allows, to not only ask students for possible sources of error, but to let them test their ideas of how to minimize them.