Determining the Ksp of Calcium
Hydroxide
Carolina Distance Learning
Investigation Manual
World-Class Support for Science & Math
2
Table of Contents
Overview ............................................................................................................................ 3
Objectives .......................................................................................................................... 3
Time Requirements............................................................................................................ 3
Background ....................................................................................................................... 4
Materials ............................................................................................................................. 6
Safety .................................................................................................................................. 7
Preparation ........................................................................................................................ 7
Activity 1: ............................................................................................................................ 7
Activity 2: ............................................................................................................................ 9
Disposal and Cleanup.................................................................................................... 10
Data Tables ...................................................................................................................... 11
©2015 Carolina Biological Supply Company
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Overview
In this experiment, students will determine the solubility product (Ksp) of calcium
hydroxide, a solid that is slightly soluble in water, by titrating a sample of calcium
hydroxide with an acid. In Activity 2, a common ion will be added and the solubility of
the new solution will be determined.
Objectives
Calculate the molar solubility of a solution.
Determine the Ksp of a molecule.
Interpret the effect of a common ion on the molar solubility of a solution.
Time Requirements
Preparation.....................10 minutes
Activity 1 .........................45 minutes
Activity 2 .........................60 minutes
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Background
An important property of molecules is their solubility, or the degree to which they
dissociate in different solvents (water, alcohols, other organic solutions, etc.) Many
chemicals have some measureable solubility in a solvent. The general classifications of
solubility are insoluble, slightly soluble, or soluble. Insoluble compounds have a
concentration (or molarity) of less than 0.01 mol/L. Slightly soluble compounds have a
molarity between 0.01 and 0.1 mol/L, whereas soluble compounds have a molarity
greater than 0.1 mol/L. Solubility is determined using an equilibrium equation between
the solid molecule and its dissolved ions. An example using silver sulfide (Ag2S) is given
below, where the “s” subscript indicates a solid and the “aq” subscript indicates
aqueous ions.
+
2−
𝐴𝑔2 𝑆 𝑆 ⇔ 2𝐴𝑔𝑎𝑞
+ 𝑆𝑎𝑞
Solubility is determined by the application of an equilibrium equation. Typically, an
equilibrium equation is used to determine the equilibrium constant (Keq):
𝐾𝑒𝑞 =
[𝐴]𝑤 [𝐵] 𝑥
[𝐶]𝑦 [𝐷] 𝑧
The bracketed letters indicate the concentration of each ion in a reaction, and the
exponent is the stoichiometric coefficient of that ion. The products are on the top, and
the reactants are on the bottom:
[𝐴𝑔+ ]2 [𝑆 2− ]1
𝐾𝑒𝑞 =
[𝐴𝑞2 𝑆]1
However, only aqueous (dissolved) components are included in the equilibrium
equation. Because solubility is the equilibrium between a solid and its aqueous ions, the
overall equilibrium equation can be simplified:
𝐾𝑠𝑝 = [𝐴]𝑤 [𝐵]𝑥
𝐾𝑠𝑝 = [𝐴𝑔+ ]2 [𝑆 2− ]1
Ksp is known as the solubility product, which is the product of the concentrations of the
ions from a parent molecule.
At standard pressure and temperature (1 atm and 20°C, respectively), Ag2S has a Ksp of
6.0 x 10-51. However, the solubility of many solids increases as the temperature of the
system increases; thus, Ksp is a fixed value at a given temperature. The Ksp of Ag2S
increases with temperature, which leads to increased solubility of the salt. The solubility
product can be used to determine the maximum concentration of ions in a solution.
Using this information, we can determine the equilibrium concentration of Ag2S in a
saturated solution:
6.0 × 10−51 = [𝐴𝑔+ ]2 [𝑆 2− ]1
In this example, the silver and sulfide ions have different stoichiometric ratios. The sulfide
ion has the lower coefficient (1 for sulfide versus 2 for silver), so it is assigned the variable
“𝑥.” Because there are twice as many silver ions, the silver concentration is represented
by “2x.”
6.0 × 10−51 = [2𝑥]2 [𝑥]
Note that when solving for concentration (𝑥), in addition to the concentration of silver
being double that of sulfide, it is also raised to the second power.
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Therefore, solving for 𝑥:
6.0 × 10−51 = 4𝑥 3
1.5 × 10−51 = 𝑥 3
1.4 × 10−17 = 𝑥
The calculations indicate that in a saturated solution of silver sulfide, there are1.4 x 10-17
mol/L of sulfide ions. Thus, because there are twice as many silver ions (2x), the
concentration of silver ions at equilibrium in this solution is 2.8 x 10-17 mol/L.
The molar solubility refers to the number of moles of a molecule that will dissolve in 1 L
of solution. In the above example, the molar solubility of silver sulfide is 1.4 x 10-17 mol/L.
Molar solubility is determined by dividing the concentration of an ion by its
stoichiometric coefficient in the parent molecule.
In this investigation, you will determine Ksp of calcium hydroxide by titrating a sample of
saturated calcium hydroxide [Ca(OH)2] with hydrochloric acid. You will also determine
the concentration of each ion in calcium hydroxide and the overall molar solubility.
A titration is the precise addition of a titrant, which is a solution of known concentration
and composition, to an analyte, which is a solution of unknown concentration or
composition. In this acid-base titration, the titrant calcium hydroxide (a base) is
delivered using a syringe in order to measure the volume of base added within an error
range of (±0.1 mL).
The equivalence point of a titration is the point at which the moles of acid are exactly
equivalent to the moles of base in solution. The equivalence point is achieved when
sufficient titrant has been added to react with all of the analyte in a given solution. The
endpoint is the experimental estimate of the equivalence point. In this investigation, the
endpoint is identified by a color change in the indicator solution methyl orange. The
molarity and volume of the titrant are known, as well as the volume of the analyte. To
determine the concentration of the analyte, you must first determine the number of
moles of titrant that are needed to reach the endpoint. Next, you will use the balanced
chemical equation to calculate the moles of analyte present. Finally, you will use the
volume of the analyte to determine the concentration of the analyte.
In Activity 2, the equilibrium will be artificially shifted back towards the solid calcium
hydroxide by the addition of calcium chloride. Calcium chloride is exceptionally soluble
and will easily dissolve in water. This portion of the investigation demonstrates the
common ion effect and its impact on the equilibrium of solutions.
2+
−
𝐶𝑎(𝑂𝐻)2 𝑠 ⇔ 𝐶𝑎𝑎𝑞
+ 2𝑂𝐻𝑎𝑞
By adding additional calcium ions in Activity 2, the equilibrium shifts back towards
calcium hydroxide, meaning that there should be less hydroxide ion available to react.
Molar solubility will then be calculated for the sample under these non-standard
conditions.
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Materials
Included in the materials kit:
Syringe, 1-mL
Lime water (calcium hydroxide)
Calcium chloride
Hydrochloric Acid, 0.10 M
Methyl orange indicator
Needed from the equipment kit:
10-mL graduated cylinder
25-mL Erlenmeyer flask
Electronic balance
Disposable pipet
Needed, but not supplied:
Plain sheet of white paper
Bottled water
Reorder Information: A replacement kit for Determining the Ksp of Calcium Hydroxide,
item number 580368, can be ordered from Carolina Biological Supply Company.
Call 1-800-334-5551 to order.
©2015 Carolina Biological Supply Company
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Safety
Wear your safety goggles, chemical
apron, and gloves at all times while
conducting this investigation.
Read all of the instructions for this laboratory activity before beginning. Follow the
instructions closely, and observe all established laboratory safety practices,
including the use of appropriate personal protective equipment (PPE) described
in the Safety and Procedure sections.
Hydrochloric acid and calcium hydroxide are corrosive. In the event
of contact with skin or eyes, the affected area should be immediately
flushed with water for 15 minutes.
Calcium chloride can be harmful is swallowed and is a serious eye
irritant.
Do not eat, drink, or chew gum while performing this activity. Wash your hands with
soap and water before and after completing the investigation, and clean up the
work area with soap and water. Keep pets and children away from lab materials and
equipment.
Preparation
1.
Read through the Procedure.
2.
Obtain all materials.
3.
Answer any pre-lab questions.
Activity 1:
Titration of Saturated Calcium Hydroxide with Hydrochloric Acid
1.
Place the 25-mL flask on the balance. Record the mass in Data Table 1.
2.
Measure 3-mL of lime water with the 10-mL graduated cylinder.
3.
Pour the lime water into the flask.
4.
Record the exact mass of the flask + lime water in Data Table 1.
5.
Add 5 drops of methyl orange to 3 mL Ca(OH)2.
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6.
Fill the syringe with hydrochloric acid, drawing the plunger back with the tip of the
syringe positioned in the HCl to fill the syringe. Try to minimize the amount of air in
the syringe.
The amount of air in the syringe must be minimized. If there are any air
bubbles in the syringe, empty the syringe and refill until no air bubbles are
visible.
7.
Record the initial volume of HCl solution in the syringe to the nearest 0.01 mL in your
Data Table 1 for Sample 1.
8.
Place the flask containing 5 mL Ca(OH)2 on the white sheet of paper. The white
background will make the color change more noticeable.
Read the volume of the syringe at the edge of the plunger.
9.
Add HCl drop wise from the syringe with constant swirling of the flask, until a single
drop of HCl causes a color change that persists for a few seconds after addition.
This indicates that the endpoint has been reached.
To gain more control of single drops from the syringe, move the plunger by
holding it where it enters the syringe instead of pressing the end of the
plunger.
10. Once the endpoint has been reached, record the final volume of HCl left in the
syringe in Data Table 1 for Sample 1.
11. Calculate the total volume of HCl that was required to reach the endpoint by
subtracting the end volume from the start volume and record the volume in Data
Table 1.
12. Dispose of the titrated Ca(OH)2 sample in the sink.
13. Rinse the 25-mL flask with bottled water.
14. Repeat steps 1–13 for samples 2 and 3 and record the values in Data Table1.
15. Calculate the concentration of OH- in your sample based on your experimental
data.
16. Calculate the concentration of Ca2+, molar solubility, and Ksp
CAUTION: Complete all trials and calculations for Activity 1 before continuing to
Activity 2.
©2015 Carolina Biological Supply Company
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Activity 2:
Titration of Saturated Calcium Hydroxide + Calcium Chloride with Hydrochloric
Acid.
1.
Carefully remove the tip of the lime water bottle.
2.
Place the bottle on the electronic balance and tare the balance.
3.
Add ~0.02 – 0.04g of calcium chloride to the bottle.
4.
Reinsert the dropper tip and recap the bottle.
5.
Shake thoroughly.
6.
Allow the bottle to sit from approximately 10 minutes.
7.
Place the 25-mL flask on the balance. Record the mass in Data Table 2.
8.
Measure 3 mL of calcium hydroxide + calcium chloride with the 10-mL graduated
cylinder.
9.
Pour the calcium hydroxide + calcium chloride into the flask.
10. Record the exact mass of the flask + calcium hydroxide + calcium chloride in Data
Table 2.
11. Add 5 drops of methyl orange to 3 mL Ca(OH)2.
12. Fill the syringe with hydrochloric acid. Try to minimize the amount of air in the
syringe.
13. Record the initial volume of HCl solution in the syringe to the nearest 0.01 mL in your
Data Table 2 for Sample 1.
14. Place the flask containing the 3 mL Ca(OH)2 + CaCl2 on the white sheet of paper.
The white background will make the color change more noticeable.
15. Add HCl drop wise from the syringe with constant swirling of the flask, until a single
drop of HCl causes a color change that persists for a few seconds after addition.
This indicates that the endpoint has been reached.
16. Once the endpoint has been reached, record the final volume of HCl in the syringe
in Data Table 2 for Sample 1.
17. Calculate the total volume of HCl that was required to reach the endpoint and
record the data in Data Table 2.
18. Dispose of the titrated Ca(OH)2 + CaCl2 sample in the sink.
19. Rinse the 25-mL flask.
20. Repeat steps 7–19 for samples 2 and 3 on Data Table 2.
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21. Calculate the concentration of hydroxide in your sample based on your
experimental data.
22. Calculate the concentration of Ca2+ and molar solubility.
Disposal and Cleanup
1.
Dispose of titrated samples in the sink.
2.
Clean and dry the flask and return it to the equipment kit.
3.
The specimen jars can be washed and disposed in the trash or recycled.
©2015 Carolina Biological Supply Company
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Data Tables
Data Table 1: Titration of Calcium Hydroxide with Hydrochloric Acid
Sample 1
Mass of Erlenmeyer Flask
Mass of Erlenmeyer Flask
+ Calcium Hydroxide
Solution (lime water)
Mass of Calcium
Hydroxide solution
Volume of Ca(OH)2
Density = 1.000 g/mL
Concentration of HCl (M)
Initial HCl volume in
syringe
Final HCl volume in
syringe
Volume of HCl Delivered
Moles of HCl Delivered
Moles of OH- in Sample
Moles of Ca2+ in Sample
Molar Solubility (M)
Calculated Ksp
Average Calculated Ksp
©2015 Carolina Biological Supply Company
Sample 2
Sample 3
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Data Table 2: Titration of Saturated Calcium Hydroxide + Calcium Chloride
Name of Vinegar Sample
Sample 1
Mass of Erlenmeyer Flask
Mass of Erlenmeyer Flask
+ Calcium Hydroxide
+CaCl2 solution
Mass of Calcium
Hydroxide +CaCl2
solution
Volume of Ca(OH)2
Density = 1.000 g/mL
Initial HCl volume in
syringe
Final HCl volume in
syringe
Volume of HCl Delivered
Moles of HCl Delivered
Moles of OH- in Sample
Moles of Ca2+ in Sample
Molar Solubility
©2015 Carolina Biological Supply Company
Sample 2
Sample 3
Determining the Ksp of Calcium
Hydroxide
Pre Lab Questions
1.
Determine the Ksp of calcium fluoride (CaF2), given a molar solubility of 4.58 x
10-5 mol/L.
2.
Determine the molar solubility of potassium chromate (K2CrO4) given a Ksp of 142.
3.
When a common ion is introduced, should the molar solubility of a molecule
increase, or decrease? Why?
Discussion Questions
1.
Based on your results, could you predict the solubility of a calcium hydroxide
solution following the addition of a known amount of calcium chloride? Why or why
not?
2.
What other factors besides a common ion, can affect solubility? Design an
experiment to determine molar solubility that involves changing a different
variable.
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