A better way to use the empirical formula of magnesium oxide lab
- Brennan Koch
- 1 day ago
- 5 min read
“Wait, Mr. Koch, we already knew that!”
My student was excited to see how close her empirical formula of magnesium oxide was to the correct answer. Did she already know the formula? Yes, yes you did. And that is the magic of how chemistry builds on itself. Students could figure out the simplest ratio from looking at the valence electrons. Or they can do the reaction to see the ratio appear in mole ratios. Don’t overlook a topic as too simple when it allows students to make deeper connections to material from the past.
Here is how I used the Empirical Formula of Magnesium Oxide Lab.
Do I have a write-up? Yes. Here is a free download.
Intro Empirical Formulas
My students hadn’t even heard of empirical formulas before the lab. I gave them a crash course into the most reduced mole ratios of elements. I put a few examples on the board and had them cross off the ones that weren’t empirical. NaCl, CaCl2, and H2O could stay. H2O2 and C6H12O6 had to go.
Then I showed them the goal of the lab. We were trying to solve for MgxOy. Even writing it like that made it seem complex and really “sciency”. It also slowed them down from just saying that they knew the right answer.
Do a magnesium burning demo
I take a strip of magnesium and show the students what it looks like as it burns. I hold the Mg with crucible tongs and burn it, while looking away. If I hold really still, the strip of magnesium oxide will hang there. I ask the students to guess with their thumbs if the mass of the powder is higher (thumbs up), lower (thumbs down), or the same (thumbs to the side) as the mass of the original magnesium. You might be shocked how varied the answers are. But that gives me a chance to talk about how many moles of magnesium there are and what had to be added to form magnesium oxide. I ask the students if I caught all the mass in the strand. Of course, I did not as there is a large cloud of smoke that rolled away. I describe how we will do the reaction more slowly and more contained in order to account for all the mass.
Draw a Lab Map
I draw a lab map for every lab. It is a visual representation of what they will be doing. I can highlight dangerous steps (like not setting raging hot crucibles on my counter), places they need to record measurements, and unique methods. They also have a written lab instruction sheet, but I have found that a picture and a list of instructions increase the success rate.
My savvy AP chem and AP bio students take a picture of each lab map and insert it into their digital lab book. That way they have a model to use on their lab practical final. It’s created some good habits. Here is my lab map for this lab. I redraw this on the board each class and walk them through step by step.
Pro-Tip #1: Have students sand the magnesium strip before they weigh it in order to get a more complete and fast reaction. I use little strips of 120 grit sand paper.
Pro-Tip #2: Show the students to correct height to set their ring stand BEFORE they light the Bunsen burners. They really need it to be just above the inner cone to get the max heat. Moving it after the ring is hot is less than ideal.
Pro-Tip #3: Use the campfire test to determine if the reaction is done. If they open their crucible and blow ever-so-gently on the reaction, they can see if it springs back to life. If there are little embers like a campfire that increase their brightness, then the reaction isn’t done. If the magnesium oxide just sits there, then it’s done.
Collect all data
The students collect and record all the data. This lab only takes about 25 minutes of actual lab time. Once they have the mass of the crucible and lid, mass of the magnesium, and mass of the crucible with magnesium oxide, they clean up the lab and regroup to do calculations.
Use a picture equation to model the calculations
This was a new experiment for me this year, and it worked great. After the students had regrouped in the classroom for calculations I drew a picture equation on the board and had them solve it. It looked like this.
They had solved for the mass of the oxygen added in the reaction. Tying the physical properties of the lab to a picture and to math seemed to help the lower students understand what we were trying to solve. The higher kids already knew it, but it helped to close the math gap.
Calculate the empirical formula
Finally, the students convert their grams of magnesium to moles and their grams of oxygen to moles. I am using this lab in the middle of the mole chapter, so it has direct relation to what we are currently learning. Then they make a “Raw Answer” which is the decimal form of their ratio. They find this my dividing moles of O by moles of Mg. The winners this week got a ratio of 0.97 mol O to 1 mol Mg!
I have all the groups put their unrounded ratios on the board and then we average the class. Generally speaking, the average is pretty good. Are there crazy answers out there? Yep. But the main concept can still be taught. Once we look at the average, we discuss why it would form a 1:1 ratio. This connects it back to the periodic trends chapter with valence electrons. And it was this moment that I got the quote from the top of this article. They already knew that it should be a 1:1 ratio! But in their minds, valence electrons were in the last chapter. This chapter is about moles. But allowing them to see a real tangible connection between the concepts was amazing.
I can tend to overlook some labs and activities as “too easy”. But to the students, creating this elementary connection within chemistry helps them build a more complete understanding. And that is really where the magic is.
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