Stoichiometry is really stinking hard. When I was a new teacher I struggled with my ability to help students understand stoichiometry. I struggled with it BIG TIME as a student. Plus I was never really good at math (even though I made it up to differential equations…) Either way I was kind of nervous about it.
And the first year I did it went just okay. But after learning from those first year students. (And then learning more and more as the years passed) I now feel great about teaching stoichiometry. In fact, it’s actually one of my favorite units to teach. We often call it the “unit of food” because I bring in quite a bit of stuff for them. Outlined are the lessons that I teach, some activities that go with them and how I like to explain each concept. All lessons are taught with slides and guided notes.
Mole Concept
Of course the first step to help students understand stoichiometry is the mole concept. I’ve done this counting by weighing lab for quite some time, but last year was the first time I did it as an inquiry lab and I’ll never go back. I have the students measure a bunch of things they can’t see. I collect same-size bundles of items and hide them in plastic bags. Items like crayons, binder clips or pennies. With a little bit of information, the kids need to determine the mass of just one of the items in the bundle or the number of items in the bundle. Sometimes both!
Doing this as an inquiry lab was such a great experience. My students really benefited from doing this before ever learning the mole concept. You can learn more of the details in this Youtube video on how I set it up.
After we do this lab, moving into the mole concept really isn’t so bad. When we do the inquiry lab first, this lesson is almost TOO EASY for the students (which I love to hear!) When I go to teach them the mole, really all they need is the size of the “bundle.” Which we learn is a very large number because atoms are super small.
I have the kids learn molar mass by filling in what we call a QMT chart. That stands for Quantity – Mass – Total. It’s a very detailed way for them to fill in the molar mass in a methodical way. It helps quite a bit when we get to percent composition. We practice molar masses using this pixel art activity.
Percent by Mass
To begin teaching percent by mass, I give my students an Oreo cookie and ask them to find the percent by mass of cream in the Oreo. This is really to point out the difference between percent by mass and percent by quantity. I find that only a few of my students struggle with this concept. Then we move on to finding the percent by mass of cream in a ‘Double Stuf’ Oreo. For years and years, the Double Stuf Oreo did not actually have double the amount of cream. This made my students quite infuriated. Some students even take it so far to send a strongly worded letter to Nabisco.
We run through a quick lesson, which really is a synthesis of the lab activity. And the students are able to quickly move on to the pixel art activity to practice. This lab is currently available exclusively in my Full Year Chemistry Lab Book.
Hydrates
When it comes to hydrates there is a much more detailed lesson. I teach the students what hydrates are, how to name them, and the lab procedure for dehydrating them. This lesson also has a pixel art activity for practice. The best part is the following day, where we do the bubblegum hydrate lab. When I first started I was not lucky enough to have a full chemistry lab. So for that reason, my hydrate lab was bubble gum instead. The bubblegum represents be hydrated salt, the sugar within it represents the water. The gum wrapper represents the crucible, and the chewing takes the place of heating the hydrate. This lab always has a lot of error; like usually around 40% error. The reason is because a lot of saliva is added to the gum. The important thing here is going through the motions of the hydrate lab math.
Gram and Mole Conversions & Mole Ratios
Now we’re really getting into stoichiometry. I make sure to teach these skills separately. I have found that the full process of stoichiometry is a lot easier when students begin learning the steps piecemeal. When we talked about gram and mole conversions, I like to relate it two boxes of donuts. A box of donuts usually contains a dozen, or 12 donuts. If you had three boxes, you’d have 36. But if one of those boxes was not filled completely, and you couldn’t look inside to count them, you could take a guess as to how many donuts are in the box using the weight. And we do that same thing for atoms and moles! Sometimes we don’t work with an entire mole of substance. That’s where those ratios come into play. You guessed it, this lesson is followed up by some pixel art practice.
We also have a lab activity that gives the kids a lot of practical experience with converting grams and moles in a lab setting.
When I talk about mole ratios, I like to think of doubling or halving a recipe. If a chemical equation is just a recipe, what we’re looking to do is change the number of moles of one of the substances in that recipe. We’re really just changing the amount! Again we use a ratio to make that happen. When I’m feeling generous I bring in cookies for my students as a “visual aid” for their recipe doubling. Of course this has a pixel art activity as well.
Pixel Art Activities
I spoke about these pixel art activities throughout this entire post so here’s the details. Pixel art is an educational activity within a Google Sheets file. The sheet has 20 questions that are set up so that student responses are graded immediately. Students are able to know instantaneously if their work is done correctly or incorrectly. While they are working, every answer they get correct unveiled a piece of a secret image. This keeps them engaged, it keeps them focused and it keeps them from feeling insecure. When kids see the green check box, they know that they’re working in the right way. It really saves me from things like “Miss Reavy is this right ?” It also helps, because kids can empty the sheet and resubmit answers, to get even more practice. It is certainly my primary tool to help students understand stoichiometry.
Stoichiometry
By the time we get to stoichiometry, the kids are actually very well prepared for it. All of the prep work was put in place to help students understand stoichiometry. They know all of the skills, they’ve gotten loads of practice, and now it’s time to just put those skills to work. I give him a quick 4 steps for solving stoichiometry problems.
- determine and balance the chemical equation
- change the given to moles
- compare what you know to what you want using the mole ratio and the balanced equation
- change those moles back to grams
and with that, stoichiometry is not that intimidating. In fact getting to this point usually is pretty fun! You can get a free copy of this mole map I give to help students understand stoichiometry. We do come back to stoichiometry in the gas unit as well as the solution unit. I like to keep those separate, since this is SUCH a beast to understand.
