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Experiment 22 - Concentration of Solutions
The purpose of this experiment is to acquaint you with the concept of
concentration of solutions. Although there are a variety of ways of describing a solution’s
concentration, we will use just one system in this experiment, that of “molarity,” or
“moles per liter,” abbreviated mol/L or M. The experiment will also give you a chance to
see how differences in concentration affect the behavior of a solution, making it react
faster or slower, and making it able to react with more or less of some other substance
(for the same volume of solution).
“Prepare a tenth of a liter (0.10 L) of a 1 M solution of CuCl2•2H2O.” What,
exactly, does this statement mean? First of all, a tenth of a liter needs to be expressed as a
certain number of milliliters, since our laboratory graduated cylinders are marked off in
milliliters. You will be making, then, 100 mL of solution.
What does the “1 M” mean? It means one mole of the material that you are going
to dissolve – in this case, CuCl2•2H2O – would be present in one liter of a solution that
has the concentration described as “1 M.” (M really says, “moles of dissolved material
per liter of solution.”) However, we will not need a full liter, so we will make only 0.10 L
(100 mL). Question: If you need 170.05 grams (which is one mole of CuCl2•2H2O) to
make one liter of 1 M solution, then how much will you need to make just one-tenth as
much solution? grams, or moles.
Safety Precautions:
• Wear your safety goggles.
Waste Disposal:
• All solutions from this experiment should be collected and placed in an inorganic
waste bottle (with a blue label) in one of the fume hoods.
Procedure - Part 1: Preparing solutions of known concentration
1. Prepare 100 mL of 1 M CuCl2•2H2O solution by weighing out exactly 17.00
grams of the crystals (the instructor will explain how we can weigh it so exactly),
then transferring them from the weighing paper into a 100-mL graduated cylinder.
Dissolve them by adding about 50 mL of deionized water and mixing. Then, add
more deionized water to bring the volume up to the 100-mL mark. Finally, mix
the solution thoroughly by pouring it back and forth into a clean beaker. Label the
graduate “1 M.”
2. Pour enough of the solution into a dry test tube that it stands about 2 inches deep.
Label the tube “1 M” also. Save it in your test tube rack. Also save the graduate
with the remaining 1 M solution.
3. Next we will turn some of the 1 M solution into a 0.5 M solution. Question: How
does the concentration compare – is a 0.5 M solution twice as concentrated as a 1
M solution, half as concentrated, or what? Logically, then, to make a 0.5 M
solution from a 1 M solution you would need to do what to the 1 M solution – add
water, add more CuCl2•2H2O, or what?
Pour 50 mL of the 1 M solution from the graduate into a second 100-mL graduate,
then carefully add enough water to bring the solution up to the 100-mL mark. Mix
by pouring back and forth between a dry beaker and the graduated cylinder. Label
the graduate “0.5 M.”
4. Pour enough of this 0.5 M solution into a test tube that it stands at the same depth
as the liquid in the first test tube. Label this test tube “0.5 M.”
