≡ Menu

Chemistry Problem Breakdown: Molarity

In this video I do a chemistry problem where we go through the relationship between molarity, moles and molecular mass solving a molarity problem that I’ve pulled from a general chemistry midterm exam.

On the surface it seems like a very straightforward problem. There’s not a lot of work involved here, but moles, solutions, atomic mass, looking up stuff in the periodic table can be, if you’re not that solid on the stuff, it can get very confusing very fast. Let’s jump in.

Download the full Molarity problem breakdown and transcript PDF you can take with you

Problem Statement

Calculate the mass of KI in grams required to prepare 5.00 x 10^2 mL of a 2.80 M solution.






0:41 – Breaking down the problem statement

0:56 – Dissecting how we got the final answer: In our answer here, what we’ve got is three different things multiplied together: 2.80 molar, 0.5 L, and 166 grams per mole. What do these mean? Where do they come from? Why are we multiplying them together?

2:15 – How we determine the number of grams/mole of KI: Looks like this 166 grams is coming from adding together 39.1 and 126.9, which come from the atomic mass of Potassium and Iodine. Where do we get these and why are we adding them together?

2:38 – What “molar” means and how it’s used in this problem: We know this is a 2.8 molar solution, and up here in the step before we have this in moles per liter. That gives us a hint that molar is actually the number of moles per liter of a solution. That’s just telling you how many molecules of this KI are dissolved in 1 liter of this water solution.

3:57 – A deeper dive into moles and molarity: From Wikipedia, we have a definition of a mole, and it’s saying that a mole is a unit of measurement for the amount of a substance. That means that it’s representing a number of molecules, and it’s saying that the number’s expressed by the Avogadro constant, which represents a fixed quantity of molecules. Molarity is the number of moles dissolved per liter of solution.

4:14 – How molecular mass relates to the mass of 1 mole of a molecule: The mass of 1 mole of a substance in grams is equal to the molecular mass of the substance. For example, the mean molecular mass of water is 18.015, therefore 1 mole of water has a mass of 18.015 grams.

5:55 – How to use the Periodic Table to determine the grams/mole of KI (potassium iodide): Looking at the top number given, the atomic mass of potassium is 39.1, and the atomic mass of iodine is 126.9, which is exactly what we’re plugging in in this equation or our problem.


Okay that’s a Reverse Learning breakdown of finding mass of a set of molecules dissolved in water at a given molarity.

You can also view the video above on Youtube.

Leave a Comment