# Molarity

## Lesson Plan

### Objectives

In this lesson, students learn about how molarity is used in preparing solutions with specific molar concentrations in the laboratory. Students will:

• describe how concentration of solutions can be expressed in molarity.
• calculate molarity, volume, and number of moles of solutions and relate these variables to each other.
• prepare solutions with a given molarity.

### Vocabulary

• Molality (m): The number of moles of solute in each kilogram of solvent. m = mol solute/kg solvent.
• Molar Solution: A solution that expresses concentration in moles per Liter (mol/Liter).
• Molarity (M): The number of moles of solute in one liter of solution. M = mol/Liter.
• Solute: The dissolved substance in a solution.
• Solvent: The major component in a solution, in which the solute is dissolved.

### Duration

45 minutes/1 class period

### Prerequisite Skills

Prerequisite Skills haven't been entered into the lesson plan.

### Materials

• For each lab group:

o   KMnO4, potassium permanganate

o   100-mL beaker

o   scales or triple-beam balances

o   eyedropper

o   distilled water

o   goggles

o   non-latex gloves

o   lab aprons

### Related Materials & Resources

The possible inclusion of commercial websites below is not an implied endorsement of their products, which are not free, and are not required for this lesson plan.

• For each lab group:

o   KMnO4, potassium permanganate

o   100-mL beaker

o   scales or triple-beam balances

o   eyedropper

o   distilled water

o   goggles

o   non-latex gloves

o   lab aprons

### Formative Assessment

• View
• Collect and assess each student’s step-by-step procedure for preparing a 0.4 M solution of KMnO4 from the lab activity.
• Collect and assess the Molarity Calculations worksheet.

### Suggested Instructional Supports

• View
Scaffolding, Active Engagement, Modeling, Explicit Instruction W: This lesson introduces how molarity can be used as a standard of concentration for solutions. H: The lesson engages students in thinking about concentration by asking them how they could adjust the strength of iced tea. E: This lesson takes place in the lab. It begins with a demonstration on preparing a molar solution, then students practice preparing molar solutions in lab groups, and then they complete a worksheet independently. R: Students practice the concept of molarity as they solve the problems on the worksheet. E: Students express their understanding by participating during the instruction time and the lab activities, and by completing the follow-up questions on the worksheet. T: This lesson can be tailored by giving students a list of steps for preparing a molar solution and by having students order the steps for preparing a molar solution. O: The lesson begins with a warm-up to focus students on concentration, and then the teacher demonstrates the concept of molarity by preparing a solution and writing the steps on the board, and then students practice independently in hands-on activities and on a worksheet.

### Instructional Procedures

• View

To prepare, gather all materials for this lesson ahead of time. This lesson will take place in the laboratory.

Write the following questions on the board, and have students answer them in their notes. “You are preparing iced tea for your family. Your grandmother likes to drink it extra strong. How could you prepare one pitcher that is regular strength and one that is double-strength?” (Add the same amount of water to each pitcher, but add twice as much tea to one pitcher to make it double-strength.)

Tell students that chemistry teachers must prepare some of the solutions that students use in laboratories from highly concentrated solutions. Show them a bottle of a concentrated solution and ask, “How do you think we get the right concentration (strength) to use in the lab?” If needed, review the terms solute and solvent at this point.

Explain the concepts of molarity and molar solutions. Have students help you demonstrate the preparation of a molar solution. Prepare 1.00 liter of a 1.0 M solution of KMnO4, potassium permanganate. Have a student actually prepare the solution and another student write the calculations on the board. Note: Potassium permanganate will stain any organic material, including skin, clothing, and paper.

Step 1: Find the formula weight of KMnO4. (158.04)

Step 2: Find the mass of KMnO4 in one mole.

 1 mol KMnO4 × 158.04 g = 158.04g KMnO4 1 mol KMnO4

Step 3: Prepare the solution. In a beaker, add distilled water 158.04 g KMnO4 to make one liter. Note that the total volume of the solution is one liter, and not the volume of water added to the KMnO4.

Tell students that potassium permanganate is often used in the film and television industries to “age” props and costumes. Its oxidizing effects cause cloth, ropes, wood, and glass to look very old and worn. It was used on props and sets in the movies “Troy,” “300,” and “Indiana Jones.” It is also used as an antiseptic, for water treatment, and for synthesizing organic compounds in the lab. (Additional information is available at http://en.wikipedia.org/wiki/Potassium_permanganate.)

Ask students how you could prepare 1.00 liter of a 1.0 M solution of K2Cr2O7, potassium dichromate. Write the calculations on the board. If you have K2Cr2O7, have them tell you how to prepare the solution. Ask students to explain why 1.0 M of one solution is made with 158.0 g and 1.0 M of the other solution is made with 294.2 g? (The compounds have different formula weights.)

Have students work with their lab partners/groups to prepare 1.00 liter of a 0.5 M solution of KMnO4.

We already know that the formula weight of KMnO4 is 158.04.

Find the mass of KMnO4 in 0.5 mol.

 0.5 mol KMnO4 × 158.04 g = 79.02g KMnO4 1 mol KMnO4

In a beaker, add distilled water to 79.02 g KMnO4 to make one liter. Note that the total volume of the solution is one liter, and not the volume of water added to the KMnO4.

Have students write the steps for preparing a molar solution in their notes, in their own words.

Challenge students to come up with a way to prepare 250 mL of a 0.4 M solution of KMnO4, using the 0.5 M solution. Give them time to discuss their procedure and then carry it out. Have students in each group record the steps they took and collect this as a formative assessment.

Hand out copies of the Molarity Calculations (S-C-8-3_Molarity Calculations and KEY.doc). Have students complete the worksheet independently in class or as a homework assignment.

Extension:

• Students who might need an opportunity for additional learning can order the steps on the Steps for Preparing a Molar Solution worksheet (S-C-8-3_Steps for Preparing a Molar Solution and KEY.doc). Give students extra practice problems for preparing solutions of various molarities; this will also reinforce the skill of calculating molecular weights.
• Introduce the concept of molality to students who may be going beyond the standards, and have students complete the Molality worksheet (S-C-8-3_Molality and KEY.doc).

### Related Instructional Videos

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DRAFT 06/01/2011 