Moles and Molarity

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Objective

Students will be able to use the molarity formula to calculate moles in a solution. Unknown amounts of moles, liters, or molarity can be calculated when the other two are known.

Big Idea

Molarity is a measurement of concentration expressed in moles per liters.

Introduction

In this lesson students review mole-gram and gram-mole conversions and they then learn how to use the molarity formula as a means for figuring out how many moles are in a solution. This lesson is important background information as students move toward the final learning activity of the unit, which is the titration lab. Students need to be comfortable with the molarity formula in order to determine the molarity of an unknown acid or base.

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 does this because students will be balancing chemical equations which represent chemical reactions.

This lesson aligns to the NGSS Practices of the Scientist of Using mathematics and computational thinking because students will be calculating various values using different formulas.

It aligns to the NGSS Crosscutting Concept of Systems and system models because students will characterize the composition of solutions using the molarity formula, which is a mathematical model.

In terms of prior knowledge or skills, students should have already had experience using the periodic table to derive formulas masses of compounds, and they should have also had experience with gram-mole conversions. This lesson uses some class time to review these concepts, but the bulk of the material assumes that these skills have already been learned.

There are no special materials needed for this lesson.

Do Now/Activator

10 minutes

Do Now: Students begin class by reading pages 46 and 272 in their textbook in order to answer the first 3 questions on the Molarity and Neutralization Worksheet questions. This work is important review material that I would like students to look over independently. I will refer to this material during today’s lesson, and I would like students to immerse themselves in it; my hope is that they will remember this material from earlier in the year.

Activator: I ask students to share their answers to the questions from the Do Now. There is general recognition among the students about how to convert between moles and grams, but molarity is still a new concept for students. I explain that molarity is a measure of concentration, and it can be expressed in terms of moles per liters. This is analogous, I explain, to scoops of Kool-aid per glass of water. 

 

Mini-lesson and Guided Practice

15 minutes

Mini-lesson: I begin by noting that each element’s square on the periodic table contains a mass, and this mass tells us the mass of one mole of a substance. I ask the class how I would find the mass of 1 mole of H2O. Some students do not know, but a student reminds them that to do this simply add the mass of 2 Hydrogen’s and 1 Oxygen. This seems to ring a bell for most students, judging from their response. (I make a mental note to assign one of my stronger students to work with the student who transferred from another school half-way into the school year as she will need help with this skill.)

To check for understanding, I ask students to answer the following questions, and give them 5 minutes to do so. Some students help other students, and some work it out on their own. It is clear that they have familiarity with this material, but they have not used it in a while, and so this is slow going.

  • What is the formula mass of Mg(OH)2?  (58.294 g)
  • How many moles are in 11.2 grams of Mg(OH)2? (0.192 moles)

Once students have done this work, I provide some context by relating the work we are doing today with the work that is coming up in the near future.  Students will be performing stoichiometry in an upcoming lab. Stoichiometry uses moles as its unit—in a balanced chemical equation the coefficients are moles, and mole ratios are used to figure out how much product or reactant is created or needed. Understanding how to derive moles from solution quantities is key to this work.

At this point, I show the molarity formula again, and explain that it is possible to calculate moles if you know the concentration (M) and the amount of solution you have (ml). I note that to convert ml to L, you must divide the ml by 1,000. Students take notes during this time thanks to my encouragement.

Guided Practice: After delivering these notes, I then ask students to solve the following problem:

What is the molarity if you dissolved 11.2 grams of this Mg(OH)2 into 450 ml of water? (Hint: how many liters is 450 ml?)

I chose to have students attempt this problem type early on because I want to see what they picked up from the lecture. By grappling with a problem, students will uncover what they understand as well as questions about things they do not.

Some students are overwhelmed, so I show students what I was thinking about when I set up the problem. I note that molarity equals moles over liters, and so to find molarity, they will need to divide moles by liters. We already figured out what our moles were, so we need to divide 0.192 moles by liters. I note that the problem gave us ml, but that is ok because we can divide ml by a thousand to get L. I note that when we do our lab we will be measuring in ml, so this is good practice. The final problem should be set up as 0.192 moles/ 0.450 L, which gives us M = 0.43.

I then instruct students to complete problems 4-11 on the Molarity and Neutralization Worksheet

Application

25 minutes

Student Activity: During this time the classroom comes alive. Students are engaged in various learning strategies. Some are grappling with the work independently, some are working together to solve problems, and some are engaged in a hybrid of these styles.

During this time I work the room and answer student questions that basically involve me re-teaching or clarifying some part of the mini-lesson. Some students need assistance with finding molar mass, some need help with the role that subscripts and parentheses play in finding molar mass, and some need help in calculating moles when molarity and liters are given.

I am pleased with how class is going. Students are fully engaged in the learning process. 

Debrief

10 minutes

To wrap this lesson up I invite a student to present a problem that he has solved as shown in this debrief video. In walking around the room I feel good about what students have been able to accomplish in today’s class. This student work is typical of what students produced in class.

I do not take a measure of where each individual student is, but I did make myself available for every student who wanted my help. In the next lesson students will need to use the material from today’s lesson; a few students may find that they need to do a better job asking questions about today’s class. I am comfortable with this approach, because I know that some students do need to do a better job in advocating for themselves when they need help understanding material.