Molarity and Dilution Lab



Name: ________________________________ Per: _____ Date: ____________

Molarity and Dilution Lab

Introduction:

Solutions are an important part of chemistry. In this lab you will practice preparing solutions of different concentrations. The amount of solute that is dissolved in a given quantity of solvent is expressed as the concentration of the solution. A dilute solution contains only a small amount of solute in a given amount of solution. The unit chemists use most often to describe concentration of solutions is molarity. The molarity, M, of a solution is the number of moles of solute per one liter of solution.

Purpose:

- To accurately prepare a solution of known concentration (stock solution).

- To accurately dilute this solution to a desired concentration.

Materials:

CuSO4·5H2O weighing boat balance

funnel 100-mL volumetric flask 250-mL beaker

wash bottle water plastic pipet

graduated cylinder

Procedure:

Part 1: Molarity

My group has been assigned to make 100 mL of a __ M solution of CuSO4·5 H2O.

1. Calculate the moles of solute needed for your solution. Show work!

2. Calculate the number of grams of solute needed to make your solution. The molar mass of CuSO4·5 H2O is 249.66 g. Show work!

3. Using the balance, weigh out the amount you calculated in #2.

4. Put the CuSO4·5 H2O in a 250 mL beaker. Rinse any crystals off using the H2O squirt bottle.

5. Add about 50 mL of H2O and stir.

6. Transfer the solution to the 100 mL volumetric flask using a funnel. Rinse any leftover solid or solution from the beaker into the volumetric flask.

7. Using the H2O squirt bottle and a pipet, carefully fill to the line with H2O. DO NOT OVERFILL. IF YOU DO, YOU HAVE TO START OVER.

8. Put the cap onto the flask and invert the flask several times to mix. This is your stock solution.

Part 2: Dilution

Your lab group must make 100 mL of a M CuSO4·5 H2O solution using the stock solution you made in Part 1.

1. Pour your stock solution out of the volumetric flask into the empty 250 mL beaker.

2. Using the dilution formula, calculate the number of mL of your stock solution you need. Show work!

3. Using the appropriate graduated cylinder, measure out the amount of stock solution you calculated in #2.

4. Using the funnel, pour the measured amount of stock solution into the volumetric flask.

5. Fill to the line with the H2O squirt bottle and pipet, being careful not to overfill. Put the cap on and invert the flask several times. Compare the color to your stock solution. Record the observation here.

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Label your beaker containing stock solution with its molarity. In the last 5 minutes of class, bring your beaker to the front of the room. All beakers must be arranged in order of increasing concentration.

Questions – show calculations.

1. Explain the pattern you saw in all of the stock solutions arranged on the demonstration table.

2. Calculate the number of moles required to make 100 mL of a 3.0 M solution of CuSO4·5 H2O.

3. Calculate the number of grams required to make 100 mL of a 4.5 M solution of CuSO4·5 H2O.

4. How many mL of a 2.0 M stock solution would be needed to make 100 mL of a 0.75 M solution?

5. Write a sentence explaining how you would make the solution in #4.

TEACHER NOTES

Stock M |0.01 |0.02 |0.04 |0.06 |0.08 |0.1 |0.2 |0.3 |0.4 |0.5 |0.6 | |Mass (g) |0.25 |0.5 |1.0 |1.5 |2.0 |2.5 |5 |7.5 |10.0 |12.5 |15.0 | |Dilution M |0.002 |0.004 |0.008 |0.009 |0.012 |0.02 |0.03 |0.045 |0.06 |0.10 |0.09 | |Dilution V

(mL) |20 |20 |20 |15 |15 |20 |15 |15 |15 |20 |15 | |

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