One of my very favorite parts of my life right now is the small study group I lead for an hour every Tuesday afternoon. I get 1.5 units of college credit for keeping 15 introductory chemistry students on task. I assign groups, encourage group work skills, and help them figure out how to use basic study tools to figure out answers when they get stuck. Every now and then I throw in something fancy to shake things up a little, and this was one of those weeks!

“Stoichiometry” is a fancy word for “the study of the amounts of substances in a reaction.” How much hydrogen and oxygen do you need to make water? Well, the formula is H₂O so you need two hydrogen atoms for each oxygen atom. That’s stoichiometry. Simple stuff, fancy word. It’s simple in principle, but just like math, it can layer on top of itself and become more complicated. Then again, in my opinion, stoichiometry is just a fancy word for ratio maths applied to chemical notation.

Typically, my students know to go to the front of the room, sign in, and then find their assigned seat. This week we did something completely different. I put the sign-in sheet by the door and asked them as they came in to sign in, put their things down, and come back out. As they did, I let them each select an index card. Each one was green, yellow, or red, and had “6.022 X 10²³ = 1 mole” written on one side. Chemists use moles of atoms in stoichiometry calculations in order to allow easy use of macroscopic measurements in calculations. For more information, check out Khan Academy’s video “Avogadro’s Number and Moles.” It’s 9:43 in length.

“Okay, everyone,” I said when they had gathered. “I am an artist. I make miniature sculptures out of index cards. They are small, so I sell them in bulk. In fact, I sell them by the mole! Each one of you represents my inventory. That card in your hand represents an entire MOLE of index cards! My first client wants a type of sculpture that needs one red index card, two green index cards, and one yellow index card per sculpture. Get in groups of one red, two green, and one yellow so we can see how many moles of sculptures I can make!”

The students moved around until they had made as many complete groups as they could.

“How many complete groups do we have?” I asked.

“Two,” several voices chimed in.

“How many sculptures does that mean I can make with my current inventory?”

“Two moles of sculptures!” someone said.

“YES! Exactly! And what do we have left over?”

“Green and red,” came the forlorn voices of the leftover students.

“What does that mean in terms of what limited how many batches of sculptures I could sell from my inventory?”

Blank stares all around.

“Which thing did I run out of first, and therefore meant I could not make more sculptures?”

“Yellow!” They all asserted together.

“Yes! That means yellow, if this was a chemical reaction, would be my limiting reagent.”

Noises of sudden understanding permeated about two-thirds of the group.

We ran the same drill five or six more times with different mole ratios, and each time more of the students visibly or vocally had that lovely “Ah-ha!” moment I was hoping for. Later on in the classroom, the game served as a reference point for helping students understand the concepts in their assigned work. A three-minute game reinforced all the concepts they will be directly working with for the next month, and directly or indirectly working with for as long as they study chemistry. This was one of the best ideas I have had in a classroom capacity. I find that this success, while relatively tiny in the grand scheme of things, serves to fuel my desire to become a chemistry professor.

Here is a PDF I made with instructions for running this game. Feel free to use it in any capacity you find valuable!