Calculating Molar Mass

11 teachers like this lesson
Print Lesson


TSWBAT calculate the molar mass of substances using atomic mass.

Big Idea

Students build a deeper understanding of the mole by calculating molar mass.

Introduction and NGSS Alignment

Performance Expectation (PE)/Disciplinary Core Idea (DCI)

This lesson is aligned with HS-PS1-7, the uses of mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction; and DCI-PS1.B, the fact that atoms are conserved, together with knowledge of the chemical properties of the elements involved, can be used to describe and predict chemical reactions. Students develop an understanding of the mole as a basic counting unit in chemistry used to keep track of the amount of atoms present in matter.  In this lesson students will calculate molar mass and how it relates to stoichiometry. 

Science and Engineering Practices (SP)

HS-PS1-7 is one of the few high school Performance Expectations with the primary focus on the use of mathematics to describe a concept.  Mathematical and computational thinking at the 9–12 level builds on K–8 and progresses using algebraic thinking and analysis.  Students will use mathematical thinking as they use the periodic table to calculate molar mass based on atomic masses of elements.



10 minutes

Today students will learn how to calculate molar mass.  Before they take notes on the mole and molar mass, I like to show the BrainPOP on the mole.  This particular BrainPOP is one of the best in my opinion!  It does an excellent job of introducing the mole as a counting unit.  It also explains how scientist use the mole to explain how many particles are in a sample just by measuring its mass.  It also introduces Avogadro’s number and explains how large of a number it’s based on one mole of a substance.  It also ties into my previous lessons by comparing the mole to a counting unit like a dozen.  Finally it relates the mole to elements on the periodic table, how to calculate the molar mass of a substance and provides a several examples of practical applications of scientist use the mole using stoichiometry. 

Since I am showing this BrainPOP as an introduction I don’t handout out the quiz that comes with the video, instead we will completing it as a class using white boards.  This is a quick, easy way to see what students retained from the video and does not waste paper.  It can also provide instant feedback and stimulate conversation if students have questions about one of the answers. 

BrainPOP video requires a subscription but is well worth it because they tend to have a video for just about every topic.  It takes about 5 minutes to show and depending on the amount of questions students have, the quiz can take anywhere from 5 to 10 minutes. The video may seem a bit elementary, but the quizzes are rather difficult and rarely do students score a 10 out of 10.

When showing the quiz I focus heavily on questions 6-8 which deal with molar mass and how one mole of a substance has a different mass than another, but contains the same number of particles.  These questions make for a good transition into the notes that students will be taking today on the mole, molar mass and Avogadro’s number.


20 minutes

The mole and Stoichiometry are concepts that require some form of direct instruction followed by practice problems to teach students the concept.  In order for students to understand stoichiometry, students must first learn how to calculate molar mass.  This is one day lesson that gives students several molar mass problems to practice. 

In order to calculate molar mass students need to have the background knowledge how to calculate the number of atoms in a chemical formula. This can sometimes presents an issue because there are always a few students that still struggle with this concept. These students will need extra help if they are to master calculating molar mass.  

While I am pulling up the PowerPoint I instruct the class to take out their notebooks to take about 20 minutes of notes.  This power point has two parts to it:  One that teaches about the mole and how to calculate molar mass and one that shows students molar conversions (mass to moles and moles to molecules).  The Screencast below (slide 1-7), explains the mole, how large a mole is and how to calculate molar mass. The remaining slides will be shown the following day when students learn how do do molar conversions.


While showing the Powerpoint I provide students with enough time to write everything down which sometimes can take longer than 20 minutes.  I find that it's important that all students take notes, so while students are taking notes I walk around "nudging" any student that isn't.

Guided Practice

20 minutes

The best way for students to master molar mass is to practice a variety of problems.  Since this is a new concept I like to do guided practice.  Guided practice provides students with the opportunity practice a couple of problems and receive immediate feedback.  Students are then able to correct any mistakes and adjust their thinking.

While students are copying down the last page of notes I handout the molar mass worksheet  This is not a worksheet I created, I found it on the internet.  I want to give credit to the person who created this worksheet, it provides a variety of problems that give students a sufficient amount of practice calculating molar mass. 

My standard method for guided practice is to work through an example, have the class do the next problem and then go over the problem. After doing this a couple of times, I have students work at their own pace and post a few problems at a time so students can get immediate feedback.   While students are doing the problems I groups any students that are struggling and need help together and work with them.  Typical when students struggle with molar mass the issue stems from not counting the correct number of atoms in a chemical formula.  The best way to guide students through this struggle is practice (see Molar mass student work). 

What ever students don't complete in class will be homework and checked in for credit the following day.