Determining the Ksp of Calcium Hydroxide
Purpose: To use the method of serial dilution to estimate the solubility product constant
for the “marginally” insoluble salt calcium hydroxide.
The solubility of salts
As we have discussed in class, there is no such thing as a truly insoluble salt. When any salt
is placed in water, at least a few ions are going to separate from the ionic compound and go
into solution. As the concentration of the ions in solution increases, those ions may “find”
each other, initiating the reverse reaction in which some solid reforms. When the rates of
these two processes are identical, equilibrium has been reached, and the concentration of
ions in solution will not increase.
The solubility product constant, Ksp , allows us to quantify how much salt will dissolve before
equilibrium is reached. For example, consider the insoluble salt iron(III) hydroxide. If we
describe the process of dissolving by the equation:
Fe(OH)3 (s) Fe3+ (aq) + 3OH− (aq)
Then the equilibrium constant, Ksp , will be:
Ksp = [Fe3+ ][OH− ]
3
Knowing either the concentration of one of the ions at equilibrium or the Ksp value would
allow you the other quantity of interest.
In this experiment, we will obtain an estimate for the Ksp of calcium hydroxide. Calcium
hydroxide is one of those “annoying” insoluble salts because, as the solubility rules will tell
you, it is marginally soluble, meaning that its Ksp is small, but not incredibly small. Typically Ksp values are determined by titration or gravimetric analysis in which one would
created a saturated solution of the salt in question and then react or produce a precipitate of
one of the ions. One can then use stoichiometry to back calculate the equilibrium constant.
However, in this lab we will try to bracket the Ksp value for calcium hydroxide by direct
measurement. Essentially we will create 12 different solutions of calcium cations and hydroxide anions in increasing dilute concentrations, observing if a precipitate is formed when
each of these concentrations is mixed. At the point we see no precipitate, we will know that
the concentrations of cation and anion are too low (and hence Q is less than K), which will
result in an estimate for Ksp .
Preparing to Experiment
You should find the following materials at your station
• A 24 well plate
• A 12 well micro plate
• Several clean and empty Beral micropipets
• Distilled water
• Solutions of 0.10 M calcium chloride and 0.10 M sodium hydroxide
Procedure
The procedure here is very simple. We wish to create 12 different solutions of calcium cation
and hydroxide anion, each of different concentration. To do so we will use the method of
serial dilution, in which you take a little bit of the solution from a previous well, dilute it
with a known amount of water, and then add it to the next well. Doing this over 12 wells
will create successive solutions of lessening concentration. Specifically you should do the
following.
1. Obtain 6 empty beral micropipets
2. Fill one of the pipets with 0.10 M calcium chloride
3. Fill a second pipet with 0.10 M sodium hydroxide
4. Fill a third pipet with distilled water
5. Carefully place 5 drops of 0.10 M calcium chloride into the first well of the 12 well
microplate. Make sure before you actually drop into the plate that you can
reproduce your drop size. It is extremely important that you hold the pipet vertically
while dropping. Practice your dropping technique in the 24 well plate before actually
beginning the experiment.
6. Into the second well, add another 5 drops of calcium chloride solution. To wells twotwelve, add an additional 5 drops of distilled water.
7. Using a clean pipet suck up all the solution from well 2. Thoroughly mix this solution
by sucking and expelling the solution into the second well several times. When you
are certain the solution is thoroughly mixed, drop 5 drops of the diluted solution into
well three. Place the remaining solution back into well two.
8. Continue step 7 for all 12 wells. When you complete well 12, remove 5 drops of
solution from the well.
9. Add 5 drops of 0.10 M sodium hydroxide to each well. Stir the well with a stir stick,
and wait 3-5 minutes to observe if a precipitate forms in each well.
10. Record in the data table below the concentration of each ion in the well and well as if
a precipitate formed in the well after mixing the two solutions.
11. Clean your well plate thoroughly with some soap and distilled water. Then reverse the
procedure by first adding 5 drops of 0.10 M sodium hydroxide to well 1 and 5 drops of
distilled water to wells 2-12.
12. Like in step 8 above, add an additional 5 drops of 0.10 M NaOH to well two. Thoroughly
mix the solution with a clean pipet, place 5 drops into well 3, and return the remaining
solution to well 2.
13. Continue the previous step for all 12 wells. Again, remove 5 drops from well 12 after
mixing.
14. Add 5 drops of 0.10 M calcium chloride to each well. Stir with a clean stir stick and
observe any precipitates after 3-5 minutes. Record the concentration of calcium an
hydroxide ions in the table below, as well as note in which wells precipitates form.
Well # [Ca2+ ] [OH− ] precipitate?
1
2
3
4
5
6
7
8
9
10
11
12
Table 1: Serial dilutions of 0.10 M calcium nitrate solutions
Sample calculation of concentrations of calcium cation and hydroxide anion for
well
Well # [Ca2+ ] [OH− ] precipitate?
1
2
3
4
5
6
7
8
9
10
11
12
Table 2: Serial dilutions of 0.10 M sodium hydroxide solutions
Sample calculation of concentrations of calcium cation and hydroxide anion for
well
Based on the data in tables one and two, what is the best estimate of a range for the Ksp of
calcium hydroxide? Explain.
Look up the actual value of the Ksp of calcium hydroxide and compare to your best estimate
with a percent error calculation.
Questions
1. What is the concentration of hydroxide at equilibrium in a solution that contains a
mixture of 0.10 M sodium hydroxide and 0.20 M calcium chloride? Justify your answer
with a calculation.
2. Would the molar solubility of calcium hydroxide by higher or lower in a solution of
calcium chloride? Explain, using Le Chatelier’s principle.
3. When a precipitate of calcium hydroxide is mixed with a sufficient quantity of hydrochloric acid, the solution is seen to clear. Explain, using Le Chatelier’s principle.
Conclusion