Chemistry: Equilibrium
Inquiry Question
How do we determine the solubility product equilibrium
constant (Ksp) for lead (II) iodide (PbI2)?
Name: ___________________________
Date: ____________
Up until now, we have looked at reactions that can achieve equilibrium. In Unit 3 (Solubility), you
look at a special case of equilibrium where one of the products formed may form a solid. This
process is an equilibrium process.
Now, for some clarity, let’s look at an example. Let’s show a balanced chemical reaction of lead (II)
nitrate with potassium iodide.
𝑃𝑃𝑃𝑃(𝑁𝑁𝑂𝑂3 )2 (𝑎𝑎𝑎𝑎) + 2𝐾𝐾𝐼𝐼(𝑎𝑎𝑎𝑎) → 𝑃𝑃𝑃𝑃𝐼𝐼2 (𝑠𝑠) + 2𝐾𝐾𝐾𝐾𝑂𝑂3 (𝑎𝑎𝑎𝑎)
You will notice a solid is one of the products. Even though the reaction is lead (II) nitrate with
potassium iodide, we are shifting our “spotlight” to look only at the lead (II) iodide product. It is this
product and the equilibrium it forms between its solid form and its dissociated ion form.
Understanding the degree to which the solid will dissolve and dissociate compared to other products
can help us predict outcomes in experiments or even help drive outcomes in specific directions (ex.
choosing reactants whose products display very low solubility).
During this activity, you will determine the solubility product constant (Ksp) for PbI2 by watching a
video of an experiment.
Begin by watching this video: Determination of a Solubility Product Constant
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Chemistry: Equilibrium
Data and Observations:
1. Write the dissociation equation for lead (II) iodide. It appears in the video but you should ensure
you put your charges and subscripts in the correct place.
2. Write the Ksp expression for PbI2.
3. When determining if a precipitate will form you compare a Trial Ksp to the actual Ksp. Will you see
a precipitate when the Trial Ksp is larger than or smaller than the actual Ksp?
4. Fill in Table 1 by watching the video
Table 1: Data needed to calculate the Trial Ksp at which a precipitate begins to form
Test Tube
Volume of 0.010M
Pb(NO3)2
Volume of
water(mL) added
Volume of 0.020M
KI (mL)
Volume of water
(mL) added
Precipitate or no
precipitate at room
temperature
Temperature at which
precipitate dissolves
(0C)
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1
10.0
2
8.0
3
6.0
4
4.0
5
3.0
6
2.0
0.0
2.0
4.0
6.0
7.0
8.0
10.0
8.0
6.0
4.0
3.0
2.0
0.0
2.0
4.0
6.0
7.0
8.0
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Chemistry: Equilibrium
Calculations:
1. For each test tube you will need to calculate the Trial Ksp. In order to do this you will need the
diluted concentration of the Pb2+ ion and the I- ion. The dilution formula is shown below and the
calculations for the first test tube are done for you, as an example. The old concentrations for each
compound are found in Table 1. Hint: the new volume will always be 20.0 mL.
Dilution formula
𝐍𝐍𝐍𝐍𝐍𝐍 [ ] = 𝐎𝐎𝐎𝐎𝐎𝐎 [ ] 𝐱𝐱
𝐎𝐎𝐎𝐎𝐎𝐎 𝐕𝐕𝐕𝐕𝐕𝐕𝐕𝐕𝐕𝐕𝐕𝐕
𝐍𝐍𝐍𝐍𝐍𝐍 𝐕𝐕𝐕𝐕𝐕𝐕𝐕𝐕𝐕𝐕𝐕𝐕
Test Tube 1 - 10.0 mL of each solution
𝐍𝐍𝐍𝐍𝐍𝐍 [𝐏𝐏𝐏𝐏𝟐𝟐+ ] = 𝟎𝟎. 𝟎𝟎𝟎𝟎𝟎𝟎 𝐌𝐌 𝐱𝐱
𝟏𝟏𝟏𝟏. 𝟎𝟎 𝐦𝐦𝐦𝐦
= 𝟎𝟎. 𝟎𝟎𝟎𝟎𝟎𝟎𝟎𝟎 𝐌𝐌
𝟐𝟐𝟐𝟐. 𝟎𝟎 𝐦𝐦𝐦𝐦
𝟏𝟏𝟏𝟏. 𝟎𝟎 𝐦𝐦𝐦𝐦
𝐍𝐍𝐍𝐍𝐍𝐍 [𝐈𝐈 ] = 𝟎𝟎. 𝟎𝟎𝟎𝟎𝟎𝟎 𝐌𝐌 𝐱𝐱
= 𝟎𝟎. 𝟎𝟎𝟎𝟎𝟎𝟎 𝐌𝐌
𝟐𝟐𝟐𝟐. 𝟎𝟎 𝐦𝐦𝐦𝐦
−
𝐓𝐓𝐓𝐓𝐓𝐓𝐓𝐓𝐓𝐓 𝐊𝐊 𝐬𝐬𝐬𝐬 = (𝟎𝟎. 𝟎𝟎𝟎𝟎𝟎𝟎 𝐌𝐌)(𝟎𝟎. 𝟎𝟎𝟎𝟎𝟎𝟎 𝐌𝐌)𝟐𝟐 = 𝟓𝟓. 𝟎𝟎𝟎𝟎𝟏𝟏𝟏𝟏−𝟕𝟕
Test Tube 2 - 8.0 mL of each solution
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Chemistry: Equilibrium
Test Tube 3 - 6.0 mL of each solution
Test Tube 4- 4.0 mL of each solution
Test Tube 5 - 3.0 mL of each solution
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Chemistry: Equilibrium
Test Tube 6 - 2.0 mL of each solution
Questions:
1. According to the course formula sheet, what is the actual Ksp of lead (II) iodide?
2. Based on your calculations what is the minimum value for Ksp?
3. Based on your calculations what is the maximum value for Ksp?
4. Does the actual Ksp fall between your calculated min and max?
5. Between which two test tubes should the change from precipitate to no precipitate have occurred?
6. Can you think of a reason for this discrepancy?
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Chemistry: Equilibrium
7. Fill in Table 2 below using your Ksp calculations and the temperature numbers from the video or
Table 1. The solubility is the same as the diluted [Pb2+] you calculated.
Test tube 1
Test tube 2
Test tube 3
Temperature
Ksp
Solubility
8. What is the trend in solubility as the temperature is increased?
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Test tube 4