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Molarity of a Solution (7 Favorites)

LAB in Molarity. Last updated February 7, 2019.


Summary

In this lab, students calculate concentrations of and perform dilutions of Kool-Aid and juice solutions.

Grade Level

High school

Objectives

By the end of this lesson, students should be able to

  • Calculate the amount of solute required to create a solution of a particular concentration
  • Calculate the amount of solvent required to dilute a concentrated solution

Chemistry Topics

This lesson supports students’ understanding of

  • Solutions
  • Molarity
  • Concentration
  • Dilutions

Time

Teacher Preparation: 15 minutes

Lesson: 40 minutes

Materials

  • Balance
  • 120-mL graduated cylinder
  • Paper cups
  • Kool-Aid in 300-mL vial
  • Concentrated juice solution
  • Cold water

Safety

  • This lab should not be performed in a chemistry lab, because it involves eating and drinking.
  • Do not use regular lab ware to measure any solutions that students will consume.
  • Remind students of the following rules:
    • Pour the solution into a paper cup before drinking. The vial is for measuring and mixing only.
    • Do not pour the solid Kool-Aid back into the container if you pour out too much. Dispose of the excess in the trash can.
    • Do not drink the water in the bottles .

Teacher Notes

  • This experiment was designed to use the tropical punch flavor of Kool-Aid. It was determined that approximately 80 grams dissolved in 1.0 L of solution was the correct concentration as determined by the manufacturer. Any type of powdered drink can be substituted; obtain the amount needed to make 0.50 L and substitute this amount as the gram formula mass of the solute. The final concentration will remain 2.0 M.
  • To avoid contamination, tare the vial on a balance. Obtain the amount of solute (Kool-Aid) necessary for the preparation of the solution based on calculations. Add water to the vial using the graduated sides. Make sure the students read the metric side of the scale. Snap the vial shut and secure. Shake to obtain a homogenous mixture. Open the vial and pour the solution into paper cups.
  • Do not save the Kool-Aid from year to year. It tends to clump into hard clusters because sucrose absorbs water from the air (it is hydroscopic).
  • Small plastic vials (120 mL) can serve as the volumetric flasks for preparation of the solutions. Do not use vials that have been used for chemicals.
  • The large plastic vials should be labeled: SOLUTE molecular mass = 40
  • The concentrated solutions used in part two can be purchased in the juice section of a grocery store. Purchase the concentrate cans that require no refrigeration. Read the dilution directions on the can to make sure the dilution is three cans of water to one can of concentrate.
  • Dispense the water from sports bottles with the tops that pop up. Place the water bottles in a bucket of ice to keep them cold.
  • Eating or drinking should not be allowed in a chemistry lab. This activity should be done in a classroom other than a science room. Some suggestions are to use the cafeteria, home economics room, or another classroom that does not contain chemicals.

For the Student

Lesson

Purpose

The purpose of this activity is to:

  1. Calculate the amount of solute needed to make a specific molarity of a solution.
  2. Calculate and then dilute a concentrated solution to obtain a new molarity.

Background

By rearranging the formula molarity = moles of solute/volume of solution (in liters) the following equation is obtained: molarity x volume of solution = moles of solute. Using the molecular mass of the solute, its amount in grams can be calculated.

When a solution is prepared in the lab, a volumetric flask is used. A volumetric flask is a special flask that has been calibrated to hold a specific amount of solution.

To prepare a solution, the amount of solute needed is calculated and then obtained. The solute is placed in the volumetric flask. The flask has a mark on the neck of the bottle to indicate a particular volume. Water is then added to this mark, so the bottom of the meniscus hits the line. The flask is capped, and the solution is agitated to obtain a homogeneous mixture. The concentration of the solution is recorded on the bottle along with the chemical formula of the solute. The symbol M represents molarity, the concentration equal to the number of moles of solute contained in 1 liter of the solution.

Sometimes scientists prepare solutions that are more concentrated and they dilute them to the desired molarity. The dilution formula is

M1V1 = M2V2

The initial concentration x initial volume = final concentration x final volume

Because there is a volume quantity on both sides, you can measure the volume in either L or mL. When carrying out a dilution, the new solution can be mixed in a volumetric flask.

Example:

250 mL of a 2.0-M NaOH solution is desired. How would this be prepared?

molarity x volume (in liters) = moles of solute

2.0-M NaOH = 2.0 moles NaOH/1 liter of solution 250 mL = 0.25 L

2.0 moles NaOH/1 liter of solution x 0.25 L = 0.50 moles NaOH

0.50 moles NaOH x 40 grams/mole NaOH = 20. grams NaOH

This means to prepare 250 mL of a 2.0-M NaOH solution, obtain 20. grams of NaOH and place it in a 250 mL volumetric flask. Add water to the line on the neck. Stopper the flask and mix. The solution will have a concentration of 2.0 M, so the bottle should be labeled 2.0-M NaOH.

What if you needed 10.0 mL of a 0.50-M NaOH solution, how could you prepare this solution from the 2.0-M NaOH solution you just prepared?

M1 = 2.0 M
V1 = ?? 2.0 M * x = 0.50 M * 10.0 mL
M2 = 0.50 M x = 2.5 mL
V2 = 10.0 mL So this means 2.5 mL of the 2.0-M NaOH should be put into a 10.0-mL volumetric flask and filled to the line on the neck. That solution now has a molarity of 0.50 M.

Materials

  • balance
  • Kool-aid in 300-mL vial
  • 120-mL graduated vial
  • concentrated juice solution
  • paper cups
  • cold water

Safety

  • Do not drink the solutions from the vial. The vial is for measuring and mixing only.
  • Pour the solution into a paper cup before drinking.
  • Do not pour the solid Kool-Aid back into the container if you pour out too much. Dispose of the excess in the trash can.
  • Do not drink the water directly from the bottle.

PART I: Preparation of solutions

Procedure

In the first part of this activity, you will make four different solutions of Kool-Aid.

1 mole KOOL-AID = 40 grams

Make the following solutions. For each solution, show all calculations on a separate sheet of paper. Label each paper cup with the correct concentration. After all four solutions have been prepared, taste each solution and answer the questions.

Sample #1 Make100.0 mL of a 2.0-M Kool-Aid solution

Sample #2 Make 50.0 mL of a 4.0-M Kool-Aid solution

Sample #3 Make 60.0 mL of a 2.0-M Kool-Aid solution

Sample #4 Make 70.0 mL of a 1.0-M Kool-Aid solution

Analysis

  1. Which concentration of Kool-Aid tastes the best?
  2. In the Kool-Aid solution, what was the solvent used?
  3. In the Kool-Aid solution, what was the solute used?
  4. In sample #1 and sample #2, you should have used the same amount of solute. Explain why the two solutions tasted differently.
  5. Which of the four solutions was the most concentrated?
  6. The Kool-Aid solid is mostly sucrose. Sucrose is hygroscopic.
    1. What is the molecular mass of sucrose?
    2. What does hygroscopic mean?
    3. Because sucrose is hygroscopic, how should it be properly stored in a cupboard?
    4. Calculate the molarity of the solution if 80.0 g of Kool-Aid is dissolved in 1.0 L of solution.

Part II: Dilutions

Prelab

The concentrated juice you’ll be working with for this part has a concentration of 8.0 M.

  1. On a separate sheet of paper, calculate the final volume needed to dilute 20.0 mL of the solution to 2.0-M.
  2. Calculate the amount of concentrated solution need to produce 60.0 mL of a 2.0-M solution.

Procedure

  1. Obtain 20.0 mL of juice. Dilute it according to your prelab calculations. Place the diluted juice in a paper cup labeled 2.0 M.
  2. Obtain the amount of juice needed to make 60.0 mL of juice from your prelab calculations. Dilute it. Place the diluted juice in a paper cup labeled 2.0 M.
  3. Taste the two solutions. Answer the following questions.

Analysis

  1. In the first calculation, the initial volume of concentrate was 20.0 mL.
    1. What was the final volume of the solution after dilution?
    2. What was the amount of water that was added to obtain the final solution?
  2. Did the two solutions taste the same? Explain your answer.
  3. Define the following terms using examples from this lab and from outside the lab.
    1. Solute
    2. Solvent
    3. Solution
    4. Molarity
  4. Calculate the molarity of a solution that contains 4.0 g of NaOH in 500.0 mL of solution?
  5. What is the molarity of a solution that contains 28 g of KOH in 2.0 L of solution?
  6. If 500.0 mL of 2.0-M HCl is diluted with water to a volume of 1.0 L, what is the molarity of the new solution?
  7. How many moles of KNO3 are required to make 0.50 L of a 2.0-M solution of KNO3?
  8. Which is more concentrated: 200 mL of an 8-M NaOH solution or 500 mL of a 4-M NaOH solution?