Have you ever tried to explain what a mole is, only to watch your students’ eyes glaze over? The idea of 6.02 × 10²³ particles is so abstract that it can feel impossible to make it stick. I have a Counting by Weighing lab activity that helps students see what it means to “count by weighing” and understand why chemists don’t actually count atoms one by one.
In this simple, hands-on activity, students determine the mass of one hidden object without ever seeing what’s inside the container. It’s a perfect entry point into the mole concept because it connects an abstract idea to a concrete experience, just like we do in chemistry when we group atoms into moles.
Chemistry Background: Understanding the Mole Concept
The mole is chemistry’s way of grouping particles, just like a “dozen” means 12. One mole represents 6.02 × 10²³ atoms, or molecules.
Why do we need such a large number? Because atoms are incredibly small. Just as we might measure sand by the bucket rather than individual grains, chemists measure matter by moles instead of atoms.
Every substance has its own molar mass, which tells us the mass of one mole of that substance in grams. For example:
- 1 mole of hydrogen atoms = 1.008 g
- 1 mole of oxygen atoms = 16.00 g
- 1 mole of water molecules (H₂O) = 18.016 g
If you have 2 moles of water, that’s 36.032 g. One and a half moles? 27.024 g. Chemists use these relationships to “count” atoms by weighing samples instead of trying to tally each one individually. This is exactly the skill students develop in the Counting by Weighing Lab Activity, but on a much smaller scale. A smaller scale makes it easier to wrap your mind around the concept of counting by weighing before getting into the largeness of a mole.
How to Set Up the Lab
For this lab, students are challenged to count by weighing the sample and then find the mass of one hidden object without opening the container and handling the items inside. You’ll provide several beakers, each containing multiple identical items like crayons, pennies, or pink erasers (whatever you have). Students use an electronic balance to find the total mass, then divide by the number of items contained in the bundles to determine the mass of one object.
The point is to make a unique sized bundle that applies to all of the stations you set up. I like to call them “Reavy Bundles” which is usually 8 items. So I’ll build my first sample, which is 3 Reavy Bundles of crayons. Students have to determine that means they’re handling 24 crayons. Perhaps the next station is 0.75 Reavy Bundles of pink erasers. That’s got 6 pink erasers in it. This also helps students to see that your atoms don’t have to come in exactly a mole, but can come in fractions of moles too.
Materials Needed
- Beakers
- Paper towel, shopping bags or something similar to hide your items from view
- Identical small objects (beans, pennies, marbles, Cheerios, paperclips, etc.)
- Electronic balance
- Optional: beaker and bag mass values for more precise calculations
Student Task
- Measure the total mass of each container.
- Subtract the container’s mass to find the total mass of the hidden items.
- Determine the number of items in the container
- Divide total mass by number of items to find the mass of one.
Students then apply their methods to some post lab questions to ensure they’re fully understanding how this should play out.
My Tips for This Lab
After running this lab many times, here’s what I’ve found works best:
1. Use “Bundles” Instead of “Moles.”
I call them “Reavy Bundles” to help students think of a mole as just a counting unit. I usually pick 12 per bundle since it relates to the idea of a “dozen,” but you can choose any manageable number.
2. Hide the Contents Well.
Part of the magic is the mystery! I place the items inside grocery bags and then into beakers to keep the contents hidden. Students should not rely on sight, only mass.
3. Mix It Up with Stations.
I love running this as a stations activity. Each beaker contains a different type of object (marbles, paperclips, beans, etc.), all in some quantity of the bundles. Students quickly realize that while each “bundle” has the same number of objects, their masses are very different, just like moles of different substances.
4. Add a Challenge.
Label some beakers “3 Reavy Bundles of paperclips” or “0.5 Reavy Bundles of beans.” Students must scale their calculations, reinforcing the relationship between moles, mass, and particle quantity.
Extra Information and Closing Thoughts
Now you make be thinking, “This is great but that seems like a really simple lab activity. I’m not sure this is rigorous enough for my students.” And to some extent, I’d agree with you, but you know I’m practically the queen of making things easy for me, and rigorous (tough) for my students. You can read more in this blog post about how I increase the rigor of my labs by having my students write thorough conclusions.
This counting by weighing lab activity makes a fantastic introduction to your stoichiometry unit. If you’re introducing moles for the first time, this activity will make the concept concrete and memorable. Students love this hands on activity. And teachers love it because it builds the perfect foundation for more advanced skills, like converting between grams, moles, and particles. By the end, your students will understand that chemists don’t need to see atoms to count them. They’ll have experienced the power of counting by weighing, and that’s what makes chemistry both challenging and fun.
If you love this lab and are interested in other low maintenance labs, check out this post where I share 15 labs I love!
