• 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.

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.

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.

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).