As many chemistry teachers know, grading lab reports can be a very time-consuming task. For me, the lab report that has required the most time to grade is a stoichiometry lab that I have been doing the past couple years. Though we do at least four “formal” lab reports each year, what makes this one different is that it involves a lot more calculations and subsequent results than any of our other labs. Regardless of how well they organized their report or wrote their conclusions, their results need to be checked for accuracy. This takes time. Even after eventually being able to generally eyeball their work, it still takes more time than I would like. So, this year I finally decided to sit down and generate a tool for me to expedite this process—the stoichiometry calculator.
First, a brief background on the lab and the requirements students must meet. This will help to explain why the calculator is so useful.
Summary of Stoichiometry Lab
1. I set out 7 different compounds
Sr(NO3)2, Pb(NO3)2, KOH, NaOH, Na2CO3, K2CO3, and CaCl2
2. Using their Solubility of Compounds at STP table, students choose which two compounds they would like to combine that will form a precipitate. Since each compound does not necessarily form a precipitate with every other compound, there are a total of 10 different reactions that can be performed.
3. Once decided, my only rule is that each group must start with 2 g of the metal compound of greater molar mass.
4. Students must use their recent understanding of mass/mole conversions and BCA tables to calculate how much of the other reactant they will need to ensure no leftover reactants as well as their theoretical yield.
5. Solutions are then made and combined, precipitates are formed, filtered, and left overnight to dry.
6. The following day, their actual yield is recorded and they are able to determine their percent yield.
Calculation-Based Requirements in Lab Report
I have only included the requirements that the calculator generates here, but you can find the grading rubric for the lab in the "supporting information" below.
- Initial mass of other reactant
- Theoretical yield
- Percent yield
- Completed BCA table (moles of each species involved at different parts of the reaction)
Once lab reports have been typed, I now have approximately 160 reports, each with 1 of 10 potential reactions, to sift through. In honor of an awesome cartoon, it is at this point I say “go go gadget stoichiometry calculator!” I move the student’s report to one side of my computer screen, the calculator to the other side, and I’m ready. I read what is necessary and when I arrive at their calculations and results sections, all I need are 2 values:
- Initial mass of metal compound of greater molar mass
- Actual yield
From these 2 values, the spreadsheet will automatically calculate the required results mentioned above.
After using this for the first time this year, I managed to take what would normally be a 2-3-minute process of my own self-checking calculations and cut it down to a 10-15 second process. I find the 2 values I need, type them in, check their numbers with mine and….done. It may not sound like a big deal but even if I check a student’s calculations in 2 minutes like I used to, when you compare that to the 15 seconds it takes me to achieve the same result using the stoichiometry calculator, for 160 lab reports that is a difference of nearly 5 hours!
To be clear, I would never claim this calculator is some product of pure genius and skill—far from it. It only required some of the most basic Excel skills, knowledge of molar masses, and an understanding of the molar ratios involved for each reaction. In other words, anyone can make one! However, if you would like to download this one and modify it however you want, then at least much of the work has already been done for you. There we go again….saving time!
I should also mention that the benefits of making a stoichiometry calculator go beyond its use for grading lab reports. It didn’t take too long before I realized that I could also use it to generate stoichiometry problems with ease. Between the frequency of quick assessments, retakes, practices, examples, and generating test questions, this was huge. I could put in some random mass value for a reactant in a known reaction and whip up the answer key in less than 5 seconds.
Whether you decide to make your own or download this one, I strongly recommend it. After all, have you ever heard of a teacher complaining of having too much time on their hands?