Revisiting the Mole

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Objective

Students will be able to convert grams to moles and calculate moles using the molarity formula.

Big Idea

Understanding how to use the mole is a key part of understanding stoichiometry and applying it to solutions.

Introduction

In the last lesson students conducted a trial run of an experiment they designed to see how different variables effected the reaction rate of calcium carbonate.

In this lesson students begin a two-day review of stoichiometry with the goal of figuring out whether the carbon dioxide amounts their balance registered were reasonable, or whether there was a significant measurement error.  Today students will use the molarity formula and a gram-to-mole conversion factor to ascertain the number of moles of reactants they used.

This lesson aligns to the NGSS Disciplinary Core Idea of HS-PS1-7. Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction. [

It aligns to the NGSS Practice of the Scientist of Using mathematics and computational thinking because students will use basic math to calculate the number of moles they have in each of their reactant samples.

It aligns to the NGSS Crosscutting Concept of Energy and Matter: Flows, Cycles, and Conservation: Changes of energy and matter in a system can be described in terms of energy and matter flows into, out of, and within that system because what I am asking students to do is to begin to provide an accounting of the atoms in their reaction; the first step in that process is understanding the number of moles they have.

In terms of prior knowledge or skills, students should have already studied the mole and stoichiometry. This two-day refresher is meant to re-teach a concept that many students struggled with in the year. For a complete stoichiometry unit, please see this unit.

There are no special materials needed for this lesson, though the lesson will have more significance if students have already generated some of their own data from a reaction.

Do Now/Activator

10 minutes

Do Now: Students begin class by grappling alone with a few questions: What is the molar mass of HCl? What is the molar mass of CaCO3? I give them a hint: For each compound, add up all of the element’s masses from the periodic table.

I reason that this is a good way to start class because this is a fundamental skill that I want students to have mastered. They should have learned this in the stoichiometry unit, but they have not used it in a while. This is a good way to remind students about a critical step in stoichiometry, and I can identify who needs help once I see the results. I quickly walk around the room and get a sense for where students are with this skill.

Activator:  I then note that yesterday we talked about whether the gas released was a reasonable amount. There are 2 ways that we can tell. I ask them for one way (a student responds that if it follows the trend we predicted in the hypothesis, then this is a good indicator that the amount might be reasonable because the reaction is behaving like it was predicted. I then note that while this is a good start, it is not very accurate. I note that we can calculate the amount of carbon dioxide we could get theoretically by using stoichiometry, and that over the next two days we will have a refresher course on this topic.

I have chosen this approach because I want to clearly frame the reason I have chosen to take a break from our reactions rate work: I know that most students could use a refresher course in stoichiometry, and we need to use stoichiometry in order to assess whether our carbon dioxide data is reasonable or contains a measurement error. Only by knowing the theoretical yield derived from stoichiometry can we do this.

Mini-lesson and Guided Practice

15 minutes

Mini-lesson:  I note that today we will get smart about figuring out how many moles there are in each sample of chemical we used, because moles are how the chemical equation is written. I remind students that one was in an aqueous solution and one was in a solid.  I first hand out and discuss the calculating moles notes. I encourage students to make additional comments on the notes as needed, and I encourage students to ask questions as well.

I then conduct a think-out-loud to show how to calculate moles from the periodic table and from the molarity formula.

This instructional choice reflects my desire to clearly spell out the procedures for calculating moles. Ultimately, students will (re-)learn this material if they have time to work with the material.

Guided Practice:  To ascertain student comfort with this skill, I ask that they try to solve one problem that uses the molarity formula and one gram-to-mole conversion problem from the converting solids and liquids into moles practice problems. I then check in to see how they are doing.

I chose this particular focus so that I can identify who is ready to practice this skill and who needs additional support. 

Application

25 minutes

Student Activity: Students who do need additional support have the option of working with a classmate on the Practice problems, or getting a small group re-teaching lesson with me. Students who feel confident with the skill can either help other students or practice independently, checking with classmates to see if they have similar answers.

I want students doing this work because I believe that by practicing these types of problems they will have an increased likelihood of retaining the skill. This is important because they will need to be proficient in identifying the number of moles they have for the next class.

Catch and Release Opportunities: Once I have worked with any students who want additional teaching I walk around the room offering help on common mistakes and making sure that students are exercising perseverance if they find the work challenging. As this supporting students in math video shows, students have various math deficits that make this lesson more difficult for some students than others. Please see my reflection for more thoughts on this topic.

Debrief

10 minutes

To wrap this lesson up I ask a few content questions: 

What is molar mass and why is it important? (it is the amount of mass in 1 mole of a substance; it is important because it is a number used to convert grams to moles)

How do you calculate molar mass? (by multiplying the subscript of each atom by its molar mass from the periodic table and adding together all of these products)

How do you convert grams to moles? (by multiplying grams by 1 mole over the molar mass)

How do you know how many moles you have in a solution? (by using the molarity formula)

Ending class this way allows me the chance to revisit the key points of why we are doing what we are doing, and to give students the chance to put into words the ideas they worked with today.

As the student debrief video shows, students are starting to be able to explain how to calculate for moles using the molarity formula. Some students, like the one whose student work is shown here, are easily able to calculate molar masses and moles.