# Expressing Solutions Quantitatively

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## Objective

Students will be able to calculate the concentration of solutions quantitatively as demonstrated by taking notes on a foldable, doing whiteboards, and performing partner practice.

#### Big Idea

Solutions can be expressed quantitatively by using parts per million, Molarity, percent composition, and density.

## Introduction

In this lesson students learn how to express solutions quantitatively through taking a notes on a foldable and performing practice questions.

• This lesson does not align with any specific performance expectations with the Next Generation Science Standards; however, it is imperative that students understand solutions before learning other concepts later in the year.  In particular students are learning how to use numbers to express solution concentration so they can understand what the numbers mean when they use concentrations in chemistry.
• Because this topic is no longer tested in my district I chose to only focus on the four equations of Molarity, Parts per million, percent composition, and density.
• This lesson aligns with the Next Generation Science and Engineering Practice 5: Using Mathematics and Computational Thinking.  It does so because students are solving for concentrations of solutions with various equations.

For this lesson there are no additional resources needed.

## Engage

3 minutes

I introduce students to the lesson by telling them that we are going to continue learning about solutions.

I let them know that we have been discussing solutions qualitatively, but that it is important to understand solution concentration quantitatively as well.  Finally, I tell students that we are going to be plugging and chugging with new equations but to keep in mind the steps that we have been doing all year in terms of problem solving.

## Explain

40 minutes

In this portion of the lesson I teach students about describing solutions quantitatively by presenting the unit8 lecture 3 presentation while students fill in notes on their foldable notes paper.

• Before the lesson I make the foldable by cutting out the four sections with my paper cutter so that it is ready for students to take notes.  This movie explains how I make this.
• I start the lesson by having students write the four ways to calculate solutions on the outside of the foldable.  I show them each of the four equations on slides 2-5 of the PowerPoint and also show them how it looks on an example foldable paper.  This is a picture of what it looks like on the outside of the foldable. Here is a movie of me going over how students should be writing on the outside of the foldable.
• I then go through each of the four equations with students.  For each equation I have students fill in information about the quantity on the inside (as show in this movie) and then do an example question on the back (as I show in this movie).
• I start with density on slide 6.  While reviewing density I remind students that we learned this first semester as well.
• I then have students do the example for density on the back of their foldable while I show them the question on slide 7.  I review the answer with students on slide 8.  I make sure to remind students that they should use the same problem solving techniques that we do in every other math problem in chemistry.  This movie shows how I do this for density.  For more details on this see my reflection.
• The next quantity I review with students is Parts Per Million on slide 10.  I make sure to tell students that when they are writing the mass of the solution they may have to add together the solvent and solute.
• Students then do the example question on the back while I show it to them on slide 11.  I then review the answer with students on slide 12.
• The next equation I show students is Percent composition on slide 14.
• Students then do the example on slide 15 and I review it with them on slide 16.  I make sure that students see that they must add the solute and solvent together for this one because it only gives them grams of water, not the whole solution.
• The final equation I teach students is Molarity on slide 18.  They then do the example on slide 19 and I review the example on slide 20.

## Elaborate

30 minutes

To help reinforce the ideas from this lesson I have students do partner practice appointments.

• I then play matchmacker to ensure that all students are matched up with partners for all of their appointments.
• I then have students go to their first partner and do the first two questions (A and B).
• I give students about 5 minutes to work.
• I tell students that if they are stuck that they should look back at their lecture 1 notes.
• As students start to finish answering the questions I review the answers with students by putting one student's paper with the correct answers on the document camera for all to see.
• I then have students thank their partners and go to their next appointment
• For appointment #2 I have students do question C on their paper.
• I give students about 5 minutes to work.
• As students are completing their work I go over the answer by putting one student's paper with the correct answers on the document camera for all to see.
• The common mistake that students make with this question is not adding the water and the sugar together for on the bottom.
• This movie shows me helping a group with question C.
• For appointment #3 I have students do question D and E on their paper.
• I give students about 8 minutes to work.
• If students are stuck I give them the hint that they will need to use their periodic tables for question D.
• As students are completing their work I go over the answer by putting one student's paper with the correct answers on the document camera for all to see.
• The common mistakes that students make with these questions is not converting grams to moles for question D and not converting L to mL for question E.
• This movie shows me helping a group with question E.
• For the final appointment #4 I have students do questions F and G.  For some groups I had them skip question G if we were short on time for the lesson.
• I give students about 10 minutes to do these last two questions
• For letter F students need to be reminded to add the solute and solvent for the solution.
• This movie shows me helping a group with question F.
• For letter G a lot of students aren't sure where to start so I encourage them to work with their partner to try to figure out, but that if they are stuck that they will need to use Molarity to solve for moles first and then grams.
• As students are completing their work I go over the answer by putting one student's paper with the correct answers on the document camera for all to see.

## Evaluate

10 minutes

For the final section of the lesson I have students practice what they learned by completing a practice worksheet.

I have students start the paper in class and let them know that they will be completing for homework.  I then stamp the paper for completion at the next class and go over the answers using the answer key.

Here are some examples of student responses:

• For this first example for PPM notice how the student does not add the water and the solute for question #1.  This is a very common mistake for students in this  unit.  I try to help them stop making this mistake by reminding them to read the questions carefully and to look for whether they are told that the volume is for the entire solution or just for water.
• This student also did not put units after their answers on many of the questions which I made sure to reinforce with students that all numbers should have units after the numbers (no naked numbers!).

Student 2:

• This student got the correct answers for the questions but like the first students also forgot to put units on his answers!