LAD F.1 (pg 1 of 5)
Ksp Solubility Product for Calcium Hydroxide
Name_____________________
Introduction
Most solubility equilibrium investigated in this course involve ionic compounds as opposed to molecular compounds. Remember
that the way that we investigate the solubility of ionic compounds is opposite to the way in which we study precipitation reactions
in which the product is a solid. Consider the net ionic equation for the formation of the silver chromate precipitate:
2 Ag+(aq) + CrO42−(aq) → Ag2CrO4(s) or (ppt)
(Rx 1)
By contrast, solubility equilibrium reactions are written from the perspective of the solid reactant dissolving into ions
Ag2CrO4(s)
2 Ag+(aq) + CrO42−(aq)
(Rx 2)
The equilibrium constant for solubility equilibria is called the solubility product constant, or Ksp.
The solubility expression for Rx 2 above is:
Ksp = [Ag+]2 [CrO42−]
Note that the solid is not included in such an expression because the ionic solid’s concentration does not change no mater how
much solid is present.
PreLAD: To be done BEFORE class. Show your work on this page. Circle your final answer. This will be turned in.
1.
Write a balanced equation in the space below that would represent the solubility equilibrium for calcium hydroxide. Then
write the equilibrium expression. Calculate the molar mass of calcium hydroxide.
2.
Make a data/results table with 3 separate sections, one for each of the three different methods.
PreLAD calculations. You may find it useful to preview and follow along with the Process the Data section to help with the
following calculations:
3. Write a balanced equation in the space below that would represent the solubility equilibrium for manganese(II) hydroxide.
Then write the equilibrium expression.
4.
The pH of a saturated solution of saturated manganese(II) hydroxide was determined to be 9.84, calculate the Ksp.
a. Calculate the [OH−] that is present in the solution.
b. Calculate the [Mn2+] that is present in the solution.
c. Calculate the Ksp.
5.
After measuring out 100.0 ml of saturated manganese(II) hydroxide solution into a beaker, then drying up the water, the mass
of the residue remaining was 0.0040 g.
a. Calculate the [OH−] that is present in the solution.
b. Calculate the [Mn2+] that is present in the solution.
c. Calculate the Ksp.
LAD F.1 (pg 2 of 5)
6.
Solubility Product of Calcium Hydroxide
It took 8.1 ml of 0.0025 M HCl to neutralize 200.0 ml of a saturated manganese(II) hydroxide solution.
a. Calculate the [OH−] that is present in the solution.
b. Calculate the [Mn2+] that is present in the solution.
c. Calculate the Ksp.
Procedure Overview
In this LAD, the solubility constant of calcium hydroxide will be determined by several different methods and the results will be
compared to promote discussion about which method was the most effective. In the first method, the pH of a saturated solution
will be measured by two different tools; a color indicator and a pH probe. In the second method, a known volume of a saturated
solution of calcium hydroxide is put into a dry pre-massed beaker, and the water is driven off in the oven. In the third method, a
volume of the saturated calcium hydroxide solution will be titrated to neutralization with a known concentration of HCl.
Materials
On each Tray
• 1 buret with stand for saturated Ca(OH)2 solution
• flask with cover with calcium hydroxide solution
• 100 ml beaker with watch glass cover for 0.010 M HCl
• dropper bottles with acid/base indicators
• 2x burets with stands for acid solution
• 2x 5 or 10 ml beakers for buret covers
• 2x 50 ml beakers for procedure 2
• 2x stirring plates with stirring bars
• 2x 50 ml flasks
• 2x 30 ml beakers for indicator tests
• 2x small test tubes for indicator tests
AT Center LAB Bench
• Analytic balances (4 places after the decimal)
• Extra saturated Ca(OH)2
• Extra 0.010 M HCl
AT Front Lab bench
• pH probe
Procedure
Goggles should be worn at all times. No exceptions. Aprons are optional.
Method 1 − two versions.
A. Determine the pH of the saturated using the pH probe. Put approximately 10 ml of saturated solution into the 30 ml beaker,
then bring the solution to the front to test with the pH probe.
B. Determine the pH using the universal indicator solution. Use the same ~10 ml of saturated solution in the 30 ml beaker, and
add several drops of universal indicator. Compare with the card for an approximate pH.
Method 2 - Drying and massing
C. Mass a clean dry 50 ml beaker. Use the buret to measure a known volume of saturated Ca(OH)2 solution into the beaker
(approximately 25 ml − but be sure and record the exact volume).
D. Place the beaker into the oven overnight to allow the water to evaporate.
E. Mass the beaker with the dry Ca(OH)2.
Method 3 - Titration - two or three trials each with different indicators
F. First you need to determine the color change that the indicator will produce during the neutralization reaction. Use the labeled
plastic pipets to put a squirt of the saturated solution into the small 30 ml beaker. Add a couple of drops of the indicator, and
then add small squirts of HCl until the color changes. Use the chart on page 4 to record the color change results. Test all of
the indicators, even though you need not use them all to titrate. This will help you know what color change will signal the
endpoint of the titration. Use the table on the next page to summarize the color change results.
LAD F.1 (pg 3 of 5)
name
Solubility Product of Calcium Hydroxide
color in acid
color in base
bromphenol blue
methyl orange
pka
chemical name
4.10
3’,3”,5’,5” tetrabromophenolsulfone-phthalein
3.46 4’-dimethylaminoazobenzene-4-sulfonic acid, Na salt
methyl red
5.00
4-dimethylaminoazobenzene-2carboxylic acid
alizarin red
5.50
1,2-dihydroxyanthroquinone-3-sulfonic acid
bromothymol blue
7.30
3’,3”-dibromothymolsulfonephthalein
phenolphthalein
9.50
di(p-dioxydiphenylphthalide
G. Measure a known volume of saturated Ca(OH)2 solution into the flask (more than 5 ml but less than 10 ml − but be sure and
record the exact volume). Drop in the magnetic stirring bar, and put in several drops of the indicator. Add enough tap water to
cover the stirring bar.
H. Be sure you that your acid buret is more than half full. Record the starting volume of the HCl. Titrate until the endpoint (color
change) occurs. Record the final volume of the HCl.
I.
Repeat the titration again with the same indicator if time allows. Repeat this procedure so you test with three different
indicators.
Disposal
Solutions may be poured down the sink with plenty of water. Please be sure and catch the stirring bar. Do not allow it to escape
down the drain. Rinse your beakers and flasks with plenty of water, no need to dry them.
Processing the Data
Method 1 − two versions (same calculation method).
1.
Convert the pH to pOH.
2.
Use the formula pOH = −log[OH−] to calculate [OH−].
3.
Knowing the [OH−] and the stoichiometry of the solubility equation, calculate [Ca2+].
4.
Calculate Ksp separately for both pH values measured.
Method 2
1.
Calculate the mass of calcium hydroxide in the beaker.
2.
Add the molar mass of calcium hydroxide and then calculate the moles of calcium hydroxide that was in the volume of
saturated solution that you measured out.
3.
Calculate the molarity of calcium hydroxide that was dissolved in the saturated solution.
4.
Knowing the stoichiometry of the chemical reaction for the solubility, calculate [Ca2+] and [OH−].
5.
Calculate Ksp
Method 3
1.
Calculate the volume of HCl used to neutralize the [OH−].
2.
Since H+ and OH− combine in a 1:1 ratio, use MV = MV to calculate the [OH−].
3.
Throw out any [OH−] that appear to be complete outliers and calculate the average [OH−] for the trials that you ran with any
one indicator.
4.
Knowing the [OH−] and the stoichiometry of the solubility equation, calculate [Ca2+]
5.
Calculate a separate Ksp for each indicator used.
LAD F.1 (pg 4 of 5)
Solubility Product of Calcium Hydroxide
Post LAD Questions
1.
Look up the Ksp for calcium hydroxide in your text. Which method gave the most accurate value? Which method gave the
least accurate value? Suggest why.
2.
If the pH of the saturated solution of calcium hydroxide were measured at 35º instead of 25ºC, how would you expect the pH
and resulting Ksp calculation to be affected? Explain with a combination of thermodynamics and LeChatelier’s Principle.
(Show your calculation of the ∆Hdissolve for calcium hydroxide.)
3.
In Method 2, if the beaker were damp before taking the mass of the empty beaker, how would the resulting Ksp be affected.
(Follow your error analysis through each affected measurement and resulting calculation.)
4.
In Method 3, what would be the effect on the calculated Ksp value if there were an air bubble in the tip of the buret when you
started the titration, and this air bubble were drained during the titration? (Follow your error analysis through each affected
measurement and resulting calculation.)
LAD F.1 (pg 5 of 5)
5.
Solubility Product of Calcium Hydroxide
Suppose a saturated solution of calcium hydroxide were stored in an open container at a constant temperature and half of the
water evaporated.
a. Would the Ksp tested after the evaporation occurred be larger, smaller, or remain the same? Justify your answer.
b.
What changes would you observe in the solution in the open storage container after the water evaporated?
6.
Why is it best to store the saturated solution of calcium hydroxide with some solid present on the bottom of the container?
7.
Many orange juices that are fortified with calcium contain a mixture of calcium hydroxide, citric acid, and maleic acid. How
does including these two acids increase the molar solubility of the calcium hydroxide, allowing more calcium to be delivered
to the person who consumes the orange juice?