Teacher Notes Lab 4. Molarity: What Is the Mathematical ...• Solutions NOS and NOSI concepts...

70 National Science Teachers Association

LAB 4

Teacher Notes

Lab 4. Molarity: What Is the Mathematical Relationship Between the Moles of a Solute, the Volume of the Solvent, and the Molarity of an Aqueous Solution?Purpose The purpose of this lab is to introduce students to the concepts of solutes, solvents, and molarity. This lab gives students an opportunity to use a computer simulation to explore the mathematical relationship between the moles of a solute, the volume of a solvent, and the molarity of an aqueous solution. Students will also learn about the wide range of methods that scientists can use during an investigation and the difference between data and evidence.

The ContentThe properties and behavior of many solutions depend not only on the nature of the solute and the solvent but also on the concentration of the solute in the solution. Chemists use many different units when expressing concentration; however, one of the most common units is molarity (M), which is the number of moles of solute per liter of solution. The mathematical relationship between molarity, moles of solute, and volume of solution is

molarity = moles of solute / liters of solution

TimelineThe instructional time needed to complete this lab investigation is 130–200 minutes. Appendix 2 (p. 501) provides options for implementing this lab investigation over sev-eral class periods. Option C (200 minutes) should be used if students are unfamiliar with scientific writing because this option provides extra instructional time for scaffolding the writing process. You can scaffold the writing process by modeling, providing examples, and providing hints as students write each section of the report. Option D (130 minutes) should be used if students are familiar with scientific writing and have the skills needed to write an investigation report on their own. In option D, students complete stage 6 (writing the investigation report) and stage 8 (revising the investigation report) as homework.

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71Argument-Driven Inquiry in Chemistry: Lab Investigations for Grades 9–12

Molarity What Is the Mathematical Relationship Between the Moles of a Solute, the Volume of the Solvent, and the

Molarity of an Aqueous Solution?

Materials and Preparation The materials needed to implement this investigation are listed in Table 4.1. The Molarity simulation was developed by PhET Interactive Solutions, University of Colorado (http://phet.colorado.edu), and is available at http://phet.colorado.edu/en/simulation/molarity. It is free to use and can be run online using an internet browser. You should access the website and learn how the simulation works before beginning the lab investigation. In addition, it is important to check if students can access and use the simulation from a school computer because some schools have set up firewalls and other restrictions on web browsing.

TABLE 4.1Materials list

Item Quantity

Computer with internet access 1 per group

Whiteboard, 2' × 3' * 1 per group

Lab handout 1 per student

Peer-review guide and instructor scoring rubric 1 per student

* As an alternative, students can use computer and presentation software such as Microsoft PowerPoint or Apple Keynote to create their arguments.

Safety PrecautionsRemind students to follow all normal lab safety rules.

Laboratory Waste DisposalNo waste disposal is needed in this lab investigation.

Topics for the Explicit and Reflective Discussion

Concepts That Can Be Used to Justify the EvidenceTo provide an adequate justification of their evidence, students must explain why they included the evidence in their arguments and make the assumptions underlying their analysis and interpretation of the data explicit. In this investigation, students can use the following concepts to help justify their evidence:

• Moles

• Concentration

• The nature of aqueous solutions



LAB 4

We recommend that you discuss these fundamental concepts during the explicit and reflective discussion to help students make this connection.

How to Design Better InvestigationsIt is important for students to reflect on the strengths and weaknesses of the investigation they designed during the explicit and reflective discussion. Students should therefore be encouraged to discuss ways to eliminate potential flaws, measurement errors, or sources of bias in their investigations. To help students be more reflective about the design of their investigation, you can ask the following questions:

• What were some of the strengths of your investigation? What made it scientific?

• What were some of the weaknesses of your investigation? What made it less scientific?

• If you were to do this investigation again, what would you do to address the weaknesses in your investigation? What could you do to make it more scientific?

Crosscutting ConceptsThis investigation is well aligned with two crosscutting concepts found in A Framework for K–12 Science Education, and you should review these concepts during the explicit and reflective discussion.

• Scale, proportion, and quantity: It is critical for scientists to be able to recognize what is relevant at different sizes, time frames, and scales. Scientists must also be able to recognize proportional relationships between categories or quantities. In this investigation, for example, students need to identify that the molarity of a solution is a ratio between the moles of solutes and the liters of solvent.

• Energy and matter: Flows, cycles, and conservation: Scientists must understand how energy and matter flow into, out of, and within an ecosystem in order to understand it and to understand how human activity can disrupt the natural balance of an ecosystem. This investigation is a perfect example of why this is important.

The Nature of Science and the Nature of Scientific InquiryThis investigation is well aligned with two important concepts related to the nature of sci-ence (NOS) and the nature of scientific inquiry (NOSI), and you should review these concepts during the explicit and reflective discussion.

• The difference between data and evidence in science: Data are measurements, observations, and findings from other studies that are collected as part of an investigation. Evidence, in contrast, is analyzed data and an interpretation of the analysis.





• Methods used in scientific investigations: Examples of methods include experiments, systematic observations of a phenomenon, literature reviews, and analysis of existing data sets; the choice of method depends on the objectives of the research. There is no universal step-by-step scientific method that all scientists follow; rather, different scientific disciplines (e.g., chemistry vs. physics) and fields within a discipline (e.g., organic vs. physical chemistry) use different types of methods, use different core theories, and rely on different standards to develop scientific knowledge.

Hints for Implementing the Lab

• Learn how to use the online simulation before the lab begins. It is important for you to know how to use the simulation so you can help students when they get stuck or confused.

• A group of three students per computer tends to work well.

• Allow the students to play with the simulation as part of the tool talk before they begin to design their investigation. This gives students a chance to see what they can and cannot do with the simulation.

• Be sure that students record actual values (e.g., molarity, moles of solute, and volume) as they use the simulation.

Topic ConnectionsTable 4.2 (p. 74) provides an overview of the scientific practices, crosscutting concepts, disciplinary core ideas, and supporting ideas at the heart of this lab investigation. In addi-tion, it lists NOS and NOSI concepts for the explicit and reflective discussion. Finally, it lists literacy and mathematics skills (CCSS ELA and CCSS Mathematics) that are addressed during the investigation.



LAB 4

TABLE 4.2Lab 4 alignment with standards

Scientific practices • Asking questions and defining problems• Developing and using models• Planning and carrying out investigations• Analyzing and interpreting data• Using mathematics and computational thinking• Engaging in argument from evidence• Obtaining, evaluating, and communicating information

Crosscutting concepts • Scale, proportion, and quantity• Energy and matter: Flows, cycles, and conservation

Core idea • PS1.A: Structure and properties of matter

Supporting ideas • Molarity• Solutes• Solvents• Solutions

NOS and NOSI concepts

• Difference between data and evidence• Methods used in scientific investigations

Literacy connections (CCSS ELA)

• Reading: Key ideas and details, craft and structure, integration of knowledge and ideas

• Writing: Text types and purposes, production and distribution of writing, research to build and present knowledge, range of writing

• Speaking and listening: Comprehension and collaboration, presentation of knowledge and ideas

Mathematics connections (CCSS Mathematics)

• Make sense of problems and persevere in solving them• Reason abstractly and quantitatively• Construct viable arguments and critique the reasoning of others• Model with mathematics• Use appropriate tools strategically• Attend to precision• Look for and make use of structure• Look for and express regularity in repeated reasoning





Lab Handout

Lab 4. Molarity: What Is the Mathematical Relationship Between the Moles of a Solute, the Volume of the Solvent, and the Molarity of an Aqueous Solution?IntroductionMost of the matter around us is a mixture of pure substances. The main characteristic of a mixture is its variable composition. For example, a sports drink is a mixture of many substances, such as sugar and salt, with the proportions of substances varying depending on the type of sports drink. Mixtures can be classified as either homogeneous or heteroge-neous. Homogeneous mixtures have parts that are not visually distinguishable, whereas heterogeneous mixtures have parts that can be distinguished visually. A homogeneous mixture is often called a solution. A sports drink therefore is a solution.

Much of the chemistry that affects us occurs among substances dissolved in water. It is therefore important to understand the nature of solutions in which water is the dissolving medium or the solvent. This type of solution is called an aqueous solution. An aqueous solution contains one or more chemicals (or solutes) dissolved in water (the solvent). The most common way to describe the concentration of a solute in an aqueous solution is to use a unit of measurement called molarity. In this lab investigation, you will explore the relationship between moles of solute, volume of solvent, and molarity.

Your TaskUse a computer simulation to determine the mathematical relationship between moles of solute, volume of solvent, and molarity. Once you have determined this relationship, you should be able to set up various functions that will allow you to accurately predict

• the molarity of a solution given the moles of solute and solvent volume,

• the moles of solute given the molarity of the solution and the volume of the solvent, and

• the volume of the solvent given the molarity of the solution and the moles of the solute.

The guiding question of this investigation is, What is the mathematical relationship between the moles of a solute, the volume of the solvent, and the molarity of an aque-ous solution?



LAB 4

MaterialsYou will use an online simulation called Molarity to conduct your investigation. You can access the simulation by going to the following website: http://phet.colorado.edu/en/simulation/molarity.

Safety PrecautionsFollow all normal lab safety rules.

Investigation Proposal Required? Yes No

Getting StartedThe first step in developing your mathematical function is to determine how moles of solute and volume of the solvent are related to the molarity of a solution. The Molarity simulation (see screenshot in Figure L4.1) allows you to mix different moles of solute in different volumes of water (the solvent). It then provides a measure of the molarity of the resulting aqueous solution.

Before you start using the simulation, you must determine what type of data you will need to collect, how you will collect the data, and how you will analyze the data to answer the guiding question.

To determine what type of data you need to collect, think about the following questions:

• What type of observations will you need to record during your investigation?

FIGURE L4.1A screenshot from the Molarity simulation





• When will you need to make these observations?

To determine how you will collect the data, think about the following questions:

• What types of comparisons will you need to make?

• How will you keep track of the data you collect and how will you organize it?

To determine how you will analyze the data, think about the following questions:

• What type of calculations will you need to make?

• What type of graph could you create to help make sense of your data?

Once you have collected and analyzed your data, your group will need to develop a function that can be used to predict (1) the molarity of a solution given the moles of solute and solvent volume, (2) the moles of solute given the molarity of the solution and the vol-ume of the solvent, and (3) the volume of the solvent given the molarity of the solution and the moles of the solute. You will then need to test your function using the simulation. If you are able to use your function to make accurate predictions, then you will be able to generate the evidence you need to convince others that the function you developed is valid.

Connections to Crosscutting Concepts, the Nature of Science, and the Nature of Scientific InquiryAs you work through your investigation, be sure to think about

• the importance of looking for proportional relationships between different quantities,

• why it is important to track what happens to matter within a system,

• the difference between data and evidence in science, and

• the wide range of methods that can be used during a scientific investigation.

Initial ArgumentOnce your group has finished collecting and analyzing your data, you will need to develop an initial argument. Your argument must include a claim, which is your answer to the guiding question. Your argument must also include evidence in support of your claim. The evidence is your analysis of the data and your interpretation of what the analysis means. Finally, you must include a justification of the evidence in your argument. You will therefore need to use a scientific concept or principle to explain why the evidence that you decided to use is relevant and important. You will create your initial argument on a whiteboard. Your whiteboard must include all the information shown in Figure L4.2.

FIGURE L4.2Argument presentation on a whiteboard

The Guiding Question:

Our Claim:

Our Evidence: Our Justification of the Evidence:



LAB 4

Argumentation SessionThe argumentation session allows all of the groups to share their arguments. One mem-ber of each group stays at the lab station to share that group’s argument, while the other members of the group go to the other lab stations one at a time to listen to and critique the arguments developed by their classmates. The goal of the argumentation session is not to convince others that your argument is the best one; rather, the goal is to identify errors or instances of faulty reasoning in the initial arguments so these mistakes can be fixed. You will therefore need to evaluate the content of the claim, the quality of the evidence used to support the claim, and the strength of the justification of the evidence included in each argument that you see. To critique an argument, you might need more information than what is included on the whiteboard. You might therefore need to ask the presenter one or more follow-up questions, such as:

• What did your group do to analyze the data, and why did you decide to do it that way?

• Is that the only way to interpret the results of your group’s analysis? How do you know that your interpretation of the analysis is appropriate?

• Why did your group decide to present your evidence in that manner?

• What other claims did your group discuss before deciding on that one? Why did you abandon those alternative ideas?

• How confident are you that your group’s claim is valid? What could you do to increase your confidence?

Once the argumentation session is complete, you will have a chance to meet with your group and revise your original argument. Your group might need to gather more data or design a way to test one or more alternative claims as part of this process. Remember, your goal at this stage of the investigation is to develop the most valid or acceptable answer to the research question!

ReportOnce you have completed your research, you will need to prepare an investigation report that consists of three sections that provide answers to the following questions:

1. What question were you trying to answer and why?

2. What did you do during your investigation and why did you conduct your investigation in this way?

3. What is your argument?





Your report should answer these questions in two pages or less. The report must be typed and any diagrams, figures, or tables should be embedded into the document. Be sure to write in a persuasive style; you are trying to convince others that your claim is acceptable or valid!



LAB 4

Checkout Questions

Lab 4. Molarity: What Is the Mathematical Relationship Between the Moles of a Solute, the Volume of the Solvent, and the Molarity of an Aqueous Solution?

1. Describe the mathematical relationship between the moles of a solute, the volume of a solvent, and the molarity of an aqueous solution.

2. Some household cleaners come in concentrations stronger than necessary for basic cleaning jobs. Jeremy followed the instructions on a cleaning bottle and mixed enough cleaner with 4.0 L of water to form a 1.0 M cleaning solution. After testing his cleaning solution, he decided he should double the concentration for a tough stain. Jeremy added the same amount of cleaner and then another 4.0 L of water to his bucket.

Using what you know about molarity, explain why Jeremy did not succeed in doubling the concentration of his cleaning solution.

3. All scientific investigations are experiments.

a. I agree with this statement.b. I disagree with this statement.

Explain your answer, using an example from your investigation about molarity.





4. Evidence is data that support a claim.

a. I agree with this statement.b. I disagree with this statement.

Explain your answer, using an example from your investigation about molarity.

5. Scientists often need to look for proportional relationships between different quantities during an investigation. Explain what a proportional relationship is and why it is important, using an example from your investigation about molarity.

6. It is often important to track how matter flows into, out of, and within a system during an investigation. Explain why it is important to keep track of matter when studying a system, using an example from your investigation about molarity.


Teacher Notes Lab 4. Molarity: What Is the Mathematical ...• Solutions NOS and NOSI concepts...

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