It’s hard for chemistry teachers to understand the difficulty students have with stoichiometry. Students can struggle with the logical methodology of stoichiometry because they don’t find it logical or methodical. It’s more of a guess and check. Especially if they haven’t figured out how to plug Avogadro’s number into their calculator properly. Mole Tables help students visually organize their work so they can make better decisions about the problems they are solving.
If you’re like me, you’ve been doing stoichiometry since your high school chemistry class (for me, that’s almost 20 years). To me, stoichiometry is second nature. I can easily figure out where a student went wrong with their set-up. I’m a master at leading students through practice problems, either as a whole class or one-on-one. But just because I get it, doesn’t mean my students aren’t trying to hide that they don’t. They have spent years hiding in math class. What makes you think they won’t do that to you in chemistry?
What is stoichiometry?
Stoichiometry is just another word for dimensional analysis or the factor-label method. It refers, specifically, to predicting the amounts of reactants or products required for a reaction. It is limited specifically to grams, moles, and sometimes liters and particles. Students have definitely completed stoichiometry-like problems before they enter a chemistry classroom, but these new units can be extremely confusing.
Teaching Stoichiometry
Teaching step-by-step stoichiometry doesn’t have to be a chore. Many students are fine setting up a long stoichiometry problem. They understand the factor-label method. The problem comes in typing it into their calculators. “You mean you want me to type in ALL of those numbers at once?” It can pretty overwhelming.
I tell my students to skip any ones since they don’t make any mathematical changes. But for many students, that still isn’t enough. Have you ever had a student forget that they are multiplying by multiple fractions and instead treat every section as a proportion? I have. Why? I don’t know. Maybe they are learning about proportions in their math class at the same time and are having trouble switching from proportions to dimensional analysis.
Yes, you are right. It is a skill that they need. Being able to switch from one method to another is essential out in the “real world.”
Here’s what I’m getting at: there isn’t just one way to solve a stoichiometry problem. There are multiple ways to solve them. They are, in essence, all the same, but the setups look different. Dimensional analysis (factor-label) works really well for a lot of kiddos. Proportions work better for others. And yet still, having a place to record all of their tiny calculations works best for others. For that, I present to you the Moles Table!
What is a Mole Table?
Right now you might be thinking “Alright, Ms. Smarty Pants, what’s a Moles Table and why should I care?” I mean, I hope you’re not that cynical- but I work with some very cynical people who get agitated when I bring them up. They only want to teach stoichiometry one way. Sure, it’s easier for them. But I tend to get better results… (toots own horn).
BCA Tables = Moles Tables
A Moles Table is similar to an ICE chart (remember those?). It’s hard to google “moles tables” because they go by several different names. Most prevalent is the “BCA Table”. BCA stands for Before (initial), Change, and After (effect).
Students convert all values into moles and write them into the table. They then use the mole ratios given from the balanced chemical equation to convert from moles of reactants to moles of products. From the moles table, students can then convert to whatever unit is required.
The use of moles tables allows students the freedom to use factor-label or proportions to get to moles. Once your students know how to use a moles table, the rest is pretty intuitive. And bonus- your students will be better equipped to answer “what does your result mean?”.
I started replacing the term “BCA table” in my classroom with “moles table” a couple of years ago to help my students understand that only MOLE values can be written into the table.
How to Use Mole Tables
Step 1: Setting Up the Mole Table
Write the balanced equation. Under the equation, off to the left, write the letters B, C, and A, each on a different line. Create a grid by sectioning off each substance as well as the B, C, and A.
Next, record a zero in every products’ before space(s). Can you start a reaction with products? No. Also- does this reaction go to completion? If so, record zeros in the reactants’ after space(s). If you are doing limiting reactant problems, you will leave these spaces blank for now.
Step 2: Convert to Moles
Convert the given quantity to moles. Your students can complete this one-step calculation using the factor-label method or with proportions. I recommend discussing these options with your students’ math teachers. This could be a great opportunity to collaborate and support what they are learning in math.
Once the calculation is complete, record the result in the mole table.
Step 3: Complete the table using Mole Ratios
Use the mole ratios from the balanced chemical equation to convert from moles of your given to moles of your unknowns. It’s easy to see how to do this when your initial coefficient is “1”. Simply multiply your starting moles by the coefficient of your other reactant(s) and product(s).
In the change row, you will subtract the amount from the reactants and add the amount to the products. The change row isn’t super important until you get to limiting reactants, but it’s helpful to show your students that all of the reactants are used up to make the products.
For reactions that go to completion, the after row should have zeros for the reactants the amount added during the change for the products.
Step 4: Convert to whatever unit you need!
Now that you have molar information for all substances in the reaction, you can convert from moles to grams, liters, or particles. Totally depends on the question.
This is really where the magic happens. Students know that this table is full of moles, they just have to pick the correct conversion factor to get the unit they need.
Mole Tables may take up more space on the page, but students gravitate towards them because they can fully grasp what each calculation means. I have fewer “what do I do now” questions because they can use their own logic skills to figure it out.
But what about limiting reactants?
This is why I LOVE teaching my students how to use a Mole Table. Most of the steps for solving a limiting reactant problem with a mole table are the same as what I’ve described above. The difference comes in that you will convert both knowns to ONE product using the mole ratio. Whichever produces the least amount of moles is the limiting reactant. Using that reactant, convert to moles of everything else using the mole ratio.
Yes, this is the same process as factor-label. It just looks different. Also, it’s easier for students to understand why they have to complete two conversions for one problem.
Ahhhh, I love when students appreciate logic!
So, what do you think? Will you be trying Mole Tables this year? If you want to try them but think you’ll need a little help I have Stoichiometry Notes in my TPT store. They come with the traditional factor-label method AND mole tables!
Have I convinced you to give Mole Tables a try?
Honestly, I’d be surprised. My coworkers like the idea but never implement it. However, when they have struggling students they send them to me. I promise your struggling students can do stoichiometry, they just might need to do it in a different way.
And isn’t that what we’re best at? Adjusting our methods to our student needs. I encourage you to give Mole Tables or BCA tables a shot this year with your stoichiometry unit. Maybe pilot it with just one class so you can compare their confidence to the students in your other classes.
Want to know more about BCA Tables? Check out these links!
Stoichiometry with BCA (video)
Interested in more conceptual chemistry? Check out my other blog posts:
Teaching with Particle Diagrams
3 Easy and Powerful Ways to3 Easy and Powerful ways to use Particle Diagrams in Chemistry
One Response
Thank you for breaking down this concept into four easy steps. Here is blog post that might be useful for learning about how to increase engagement with multimodal learning.
https://thegamingclassroom.com/multimodal-learner/