Mole and Molar Mass

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Objective

Students will be able to explain the concept of a mole and molar mass and be able to perform mole-mass conversions through lab activities, notes, whiteboards, and practice calculations.

Big Idea

The mole is a quantity that allows chemists to convert from the atomic scale to macroscopic scale.

Introduction

In this lesson students are introduced to the idea of the Mole and Molar Mass. 

I introduce these concepts in relationship to the Periodic Table and Atomic Mass values on the Periodic Table.  This is a concept that builds on Dimensional Analysis which I introduced in Unit 1 Lesson 6.

Students are performing two lab activities, notes, whiteboards, and practice questions during the lesson.

This lesson aligns with three aspects of the Next Generation Science Standards:

  • Performance Expectation: HS-PS1-7: Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.
  • This lesson focuses on the use of the mole and one step conversions between atoms and mole and between mole and molar mass.
  • Science and Engineering Practice 5: Using Mathematics and Computational Thinking
  • This practice is utilized through students performing dimensional analysis problems.
  • Cross Cutting Concept 3: Scale, Proportion, and Quantity
  • This concept is integral to this lesson because students are asked to understand the scale of atoms and moles.  It is often difficult for students to synthesize how something that is so small that we cannot see them can be bundled using Avogadro's Number to make a Mole which we can visualize at the macroscopic level.
  • Students also use the proportions of atoms to moles, and moles to molar mass within the lesson.

In this lesson there are several sets of materials needed:

  1. For the introduction I use 3 different petri dish containers with 1 mole of sulfur, carbon, and aluminum per group.  To make these I zero the petri dish, add the molar mass of the substance (for example 32.1 grams for Sulfur), put on the top of the petri dish and then tape it up.  I then find the total mass of the whole dish with substance and subtract the mass of the substance itself.  Whatever the difference is I put on the petri dish as the mass of the container.
  2. For the whiteboards for each group I use a small personal whiteboard, whiteboard marker, and eraser.
  3. For the mole of chalk lab I give each group 1 piece of sidewalk chalk.

Engage

5 minutes

To engage students in the lesson and to review Dimensional Analysis from the previous unit I have students perform a Dimensional Analysis Problem. 

The problem is found on the second slide of the Power Point and students are expected to write their answers at the top of the student notes graphic organizer.

When students perform this problem I encourage them to try it on their own first and if they are stuck to go back and look over their notes from the previous unit.

I then have them work with their table partners to check their work  I go over the answer making sure to highlight the plug and chug technique where they underline what they know, circle what they want, use equivalence statements to get their answer.

One example of the answer is found at the top of this student work example.

Explore

15 minutes

I have found that students have a very difficult time understanding the concept of the mole so I decided to have them start to understand the idea in this exploration activity. 

In this activity students weigh out one mole of three different elements (Sulfur, Carbon, and Aluminum). 

Before the lab I weigh out the molar mass of each substance into a plastic petri dish container with the mass of the container written on top of each. 

The instructions for the lab are on the 2nd and 3rd slide of the Power Point and on the student notes graphic organizer.  Students also fill in the information and answer the questions on the first page of the graphic organizer.

In the lab students find the mass of the sample in the container, subtract the mass of the container, and write down the mass of the sample itself.  They then write down information about the element from the periodic table including the atomic symbol, number and mass.  This image shows students finding the mass of the containers.

Some students are not sure how to find the mass of the sample itself in the third column of the activity so as I walk around I tell them, "So if you found the mass of the same and container, and then the container itself, how can you figure out the mass of the sample itself"?

Afterwards they answer two questions to lead them to the idea that a mole equals the atomic mass of the substances and that to determine what to weight out I looked at the atomic mass on the periodic table.

This student paper shows one student's work for the lab and answers to the questions.

Explain

40 minutes

For this section of the notes I introduce the idea of Atomic Mass, the mole, and molar conversions.  I present the Power Point using slides 5-14 and students take notes on their graphic organizer

I begin with going over atomic mass and relating it to the activity that students performed in the Explore section.

I then introduce the idea of the Mole.  After a quick introduction that brings students through the first page of the notes I have students watch a TedEd movie "How Big is the Mole".  This is a quick 4.5 minute movie that helps students to see just how small an atom is and just how many particles are in a mole.  While watching the video I instruct students to simply pay attention because the mole is a tricky concept.

After the video I continue to introduce students to the Mole and then teach them how to perform unit conversions from the mole to atoms, and then atoms to moles.  As I teach each type of conversion I lead students through two example problems.  As I do the problems I reinforce that they use the proper dimensional analysis Plug and Chug technique.

These are the first and second pages of a student's answers to the notes outline paper.

Elaborate

40 minutes

In this section I perform two activities with students to allow them to practice molar conversions. 

1. I first have students do whiteboards with partners.  I use the mole_white_boards Power Point and give students one problem at a time.  See the reflection in Unit 1 lesson 6 regarding Partner Appointments for further details.

  • They are just one-step conversions from atoms to moles, moles to atoms, moles to mass, and mas to moles.  I tell students to "Make sure to show your work, round using significant figures, and include units." 
  • When students have their answers I have them hold them up and I either give a thumbs up or thumbs down and if they have them incorrect I have them try again.
  • The most common mistake students make is mixing up when to use the atomic mass (in front of grams) versus Avogardo's number (in front of atoms).

2. After Whiteboards I have students do a Compounds of Chalk Lab

  • This lab involves students determining the amount of compounds of chalk used to write their names on the sidewalk outside of my classroom.
  • I do this lab even though students are only learning about atoms because its a fun way for them to get engaged in the lesson.
  • I spend time helping students think through how to set up the lab and how they will do the calculation on the document camera.
  • I help students determine the Molar Mass of chalk through explaining how to add up the masses of the elements in the compound.  As I do this I tell them that they will learn this in more detail in Unit 4.
  • I also help students think through how they will do the lab and set up the calculations.  The attached picture shows the product of what this looks like after I help students.
  • For the procedure students first find the mass, they write their groups names outside, and then find the final mass. 
  • For the calculations students find the difference in mass and the use that value in grams to first convert to moles and then to atoms. 
  • Students then come up with a source of error and how it would effect their results.  To help students think of sources of error I tell students "To think of why their results might be different from other groups".  Some common answers for sources of error include the pressure of pushing on the chalk, the length of students names, weighing of the chalk on the balance, and chalk breaking so not being used for the name but not accounted for in final mass.
  • The attached lab example shows data from one student's lab.
  • I choose to use compounds of chalk as a way to help students see that although we have been learning about atoms, that when multiple atoms are placed together (as the case here with calcium carbonate) that they have formed a compound.   If in your classroom you prefer to use particles of chalk or formula units of chalk than that works as well.  This document has the same lab but uses particles instead of compounds.

Evaluate

20 minutes

For the final part of the lab students work on a homework assignment where they practice their mole conversions.  When they come to class the next day I go over the answers using the answer key.

The most common mistake that students make are:

  1. Forgetting when to use Avogadro's Number (in front of atoms) and when to use molar mass (in front of grams). 
  2. Not using parentheses when they plug their dimensional analysis problems with avogadro's number in the denominator into their calculators so getting incorrect answers.