How Do You Teach Molar Mass?

7/26/2019

When you teach molar mass, do you just throw up example problems? Or, is there a strategy behind the examples you choose?

There isn't a right way to teach students how to calculate molar mass, but there is definitely a wrong way! When you are choosing your examples, make sure that there is a reason behind that example.

Start with a simple example and then build ideas onto that example. Don't just jump in with a polyatomic ion containing compound and think that students will instantly understand molar mass if you just explain it really well. (Pssst....I did that!!!)

So, if you can't start with a polyatomic ion, what type of problem is good to start with?

Start by pulling out a simple chemical compound with just a cation and anion. No polyatomic ions or anything. Also, make sure that compound has a one to one charge ratio. One cation for one anion, otherwise you are not at the most basic problem type. Choose something like NaCl, HCl, or NaF. Don't choose MgF2, or Na2O.

Put that compound on the board and ask them what the average atomic mass is of each element. Write those numbers down as they tell you the masses. Then, add the two numbers together.

For example, I start with NaCl because it is something students are familiar with. Table salt.

Then, we write down the average atomic mass for each element. For sodium, we write down 23.99 amu and for chlorine we use 35.45 amu.

We add everything up as a class. At this point they are going, “This is easy!” and I warn them it does get harder so pay attention!

At this point I usually throw up nitric acid, HNO3. I write the polyatomic ion and hydrogen separately. I will usually even point to the nitrate ion and ask what it is. Usually the students will respond with "polyatomic ion" pretty quickly.

Students respond best if you show them how to calculate molar mass by getting the molar mass of the polyatomic ion first, and then add it to the cation.

You can download my free teaching notes on this topic here.

Going back to the nitrate, you want to reiterate that the 3 on the oxygen doesn’t apply to the nitrogen, it just applies to the oxygen. Sort of like distributing in algebra class.

Then, you add those together and get the molar mass.

Last, I put up something more complicated like aluminum sulfate. What makes this the most complicated problem type is that it has 2 cations and 3 polyatomic ions. That is hard for students to keep up with!

We go through several more steps to get the problem right.

I ask them, “How many aluminums are there? 2.

“How many sulfates are there?” 3.

Then, have them calculate, on their own, the total mass of the aluminum in the compound and the total mass of the sulfate in the compound.

As a class, go over the correct numbers for those and get the final number for the molar mass.