Chapter 18

The Solubility Product Constant

Write the net ionic equation

• Solutions of sodium chromate and barium chloride are mixed.

Ba2+ + CrO42- → BaCrO4

Ba2+ + CrO42- ↔ BaCrO4

BaCrO4 ↔ Ba2+ + CrO42-

• Since the reaction is reversible when can flip it.• Write the equilibrium expression for this reaction.

BaCrO4 ↔ Ba2+ + CrO42-

K = [Ba2+] [CrO42-]

BaCrO4 ↔ Ba2+ + CrO42-

Ksp = [Ba2+] [CrO42-]

• This is the solubility product expression

• It is used to determine the concentrations of ions in a saturated solution of a slightly soluble salt and determine whether a precipitate will form within a solution.

Write the solubility product expression for AgCl

Ksp = [Ag+] [Cl-]

Write the solubility product expression for MgF2

Ksp = [Mg2+] [F-]2

Write the solubility product expression for Ag3PO4

Ksp = [Ag+]3 [PO43-]

Molar Solubility

• Molar solubility is the maximum solubility of a substance expressed in moles per liter.

• Example: The molar solubility of silver chromate (Ag2CrO4) is 1.3 x 10-4 mol/L

The molar solubility of Hg2Cl2 is 6.5 x 10-7 mol/L.

Find the solubility product.

Ksp = 1.1 x 10-18

The molar solubility of Ag2CrO4 is 1.3 x 10-4 mol/L.

Find the solubility product.

Ksp = 8.8 x 10-12

The solubility of PbCrO4 is 4.30 x 10-5 g/L. Find the solubility product.

What’s the difference here?

The solubility of PbCrO4 is 4.30 x 10-5 g/L. Find the solubility product.

Ksp = 1.8 x 10-14

The solubility of PbCrO4 is 4.30 x 10-5 g/L. Find the solubility product.

Ksp = 1.8 x 10-14

See Appendix D (Pages A6 & A7)

What is the molar solubility PbCrO4?Ksp of PbCrO4 = 1.8 x 10-14

Molar solubility = 1.3 x 10-7 mol PbCrO4/L

[Pb2+] = [CrO42-] = 1.3 x 10-7 M

What are the ion concentrations?

What is the molar solubility Ag2CrO4?Ksp of Ag2CrO4 = 8.8 x 10-12

Molar solubility = 1.3 x 10-4 mol Ag2CrO4/L

[Ag+] = 2.6 x 10-4 M [CrO42-] = 1.3 x 10-4 M

What are the ion concentrations?

What is the molar solubility Ag3PO4?Ksp of Ag3PO4 = 1.8 x 10-18

Molar solubility = 1.6 x 10-5 mol Ag3PO4/L

[Ag+] = 4.8 x 10-5 M [PO43-] = 1.6 x 10-5 M

What are the ion concentrations?

What is the molar solubility Pb3(PO4)2?

Ksp of Pb3(PO4)2 = 3.0 x 10-44

Molar solubility = 7.7 x 10-10 mol Pb3(PO4)/L

[Pb2+] = 2.3 x 10-9M [PO43-] = 1.5 x 10-9 M

What are the ion concentrations?

I place a large scoopful of Ag3PO4 (a white crystalline solid) in a beaker of water and stir. I come back tomorrow.

What do I see in the beaker?

Did any of the Ag3PO4 dissolve?

What are the concentrations in the solution?

What is the molar solubility Ag3PO4?Ksp of Ag3PO4 = 1.8 x 10-18

Molar solubility = 1.6 x 10-5 mol Ag3PO4/L

[Ag+] = 4.8 x 10-5 mol Ag+/L [PO4

3-] = 1.6 x 10-5 mol PO43-/L

What are the ion concentrations?

Did any of the Ag3PO4 dissolve? If any does dissolve what are the concentrations in the solution?

[Ag+] = 4.8 x 10-5M [PO4

3-] = 1.6 x 10-5M

How would you describe this solution?

Saturated Solutions and Solubility

Crystallization: opposite of the solution process (solute becomes reattached to surface of crystal)

Saturated solution: no more solid will dissolve and a dynamic equilibrium exists between the solution and the undissolved solid.

Solute + Solvent Solution

ecrystalliz

dissolve

What would we observe if we added sodium phosphate solution to a beaker saturated

solution of Ag3PO4?

What would we observe if we added sodium phosphate solution to the beaker containing a

saturated solution of Ag3PO4?

Ksp of Ag3PO4 = 1.8 x 10-18

[Ag+] = 4.8 x 10-5 mol Ag+/L[PO4

3-] = 1.6 x 10-5 mol PO43-/L

[Ag+]3[PO43-]

= [4.8 x 10-5]3 [1.6 x 10-5] = ?

Why does Ag3PO4 precipitate?

Reaction Quotient

If Q = Ksp, the system is at equilibrium and the solution is saturated

If Q < Ksp, the solution is not saturated precipitate does not form

If Q > Ksp, the solution exceeds saturation and a precipitate forms

Q vs Ksp

Does AgCl precipitate when equal volumes of 1.5 x 10-5M solutions of AgNO3 and NaCl mix?

??

Does AgCl precipitate when equal volumes of 1.5 x 10-5M solutions of AgNO3 and NaCl mix?

NO

Does AgCl precipitate when equal volumes of 1.5 x 10-5M solutions of AgNO3 and NaCl mix?

200.0ml of 0.0015M MgCl2 is mixed with 100.0ml of 0.0015M NaOH. Does Mg(OH)2 precipitate?

??

200.0ml of 0.0015M MgCl2 is mixed with 100.0ml of 0.0015M NaOH. Does Mg(OH)2 precipitate?

Yes

200.0ml of 0.0015M MgCl2 is mixed with 100.0ml of 0.0015M NaOH. Does Mg(OH)2 precipitate?

AgNO3 solution is added to a solution of Na2CrO4. What precipitate forms?

AgNO3 is added to a 0.0010M solution of Na2CrO4. What is the [Ag+] when the Ag2CrO4 precipitate begins to form?

Clothing washed in water that has a manganese concentration exceeding 0.1 mg L-1 (1.8 x 10-6 M) may be stained with manganese. A laundry wishes to add a base to precipitate manganese as the hydroxide Mn(OH)2 (Ksp = 4.5 x 10-14). At what pH is [Mn2+] equal to 1.8 x 10-6 M?

Clothing washed in water that has a manganese concentration exceeding 0.1 mg L-1 (1.8 x 10-6 M) may be

stained with manganese. A laundry wishes to add a base to precipitate manganese as the hydroxide Mn(OH)2 (Ksp = 4.5

x 10-14). At what pH is [Mn2+] equal to 1.8 x 10-6 M?

Fractional Precipitation

• The process by which two aqueous substances in a solution are separated through the addition of a common ion, taking advantage of their different concentration needs (Ksp values) in order to form a precipitate.

• The ion with the ________ Ksp will precipitate first.

smaller

A solution contains 0.010 mol of KI and 0.10 mol of KCl per liter. AgNO3 is gradually added to this solution.

Which precipitate forms first, AgCl or AgI?

• This is a multiple equilibria problem.

• Strategy: If there is more than one equilibrium involved write both and solve for what you can.

A solution contains 0.010 mol of KI and 0.10 mol of KCl per liter. AgNO3 is gradually added to this solution.

Which precipitate forms first, AgCl or AgI?

A solution contains 0.010 mol of KI and 0.10 mol of KCl per liter. AgNO3 is gradually added to this solution. What is the [I ̄ ] in the solution when AgCl starts to precipitate?

A solution contains 0.010 mol of KI and 0.10 mol of KCl per liter. AgNO3 is gradually added to this solution. What percent of the

original [I ̄ ] remains in solution when AgCl starts to precipitate?

NH4Cl is added to a 750mL solution that is 0.10M in Mg2+ and 0.10M in NH3. The solution also contains a Mg(OH)2 precipitate.

Calculate the [NH4+] that is required to dissolve the precipitate.

NH4Cl is added to a 750mL solution that is 0.10M in Mg2+ and 0.10M in NH3. The solution also contains a Mg(OH)2 precipitate.

How many grams of NH4Cl were added?

We can increase or decrease the solubility of “insoluble” (slightly soluble) substances by

applying LeChatlier’s Principle.

• The Common Ion Effect

• pH

The Common-Ion Effect

• A salt is less soluble in a solution that has an ion in common with the salt.

• Calcium Sulfate (CaSO4) is “insoluble” (slightly soluble) in water.

• Write the equation that represents the solubility of calcium sulfate in water.

CaSO4(s) ⇌ Ca2+(aq) + SO42−(aq)

The Common-Ion Effect

CaSO4(s)⇌Ca2+(aq)+SO42−(aq)

• Describe the equilibrium system of Calcium Sulfate in water.

•The solution is saturated with most of the calcium sulfate in solid form.

•The [SO42−] in the solution is very

low and is equal to the [Ca2+].

The Common-Ion Effect

CaSO4(s)⇌Ca2+(aq)+SO42−(aq)

• Describe what would happen if a solution of sodium sulfate were added to the beaker.

•The [SO42−] would increase

causing the reaction to shift to the left reducing the solubility of CaSO4 causing the it CaSO4 to precipitate out of the solution.

pH

• The pH can affect the solubility of a solute in two ways: – through the common ion effect.

Al(OH)3 ↔ Al3+ + 3OH ?

• Is Al(OH)3 more soluble at a lower pH, higher pH, or does pH not affect the solubility of Al(OH)3 ?

Confused?

• What happens to this equilibrium if a strong base such as NaOH is added?

• What happens to this equilibrium if a strong acid such as HCl is added?

pH

• The pH can affect the solubility of a solute in two ways: – through the common ion effect.

or– when a salt contains a basic anion such as F–,

CH3CO2–, or CN– it is normally more soluble at

low pH’s.– When a salt contains an acidic ion such as Ag+,

Zn2+, or Al3+ it is often normally more soluble at high pH’s.

FeS ↔ Fe2+ + S2-

• What happens to this equilibrium if a strong acid such as HCl is added?

• A strong acid such as HCl will often dissolve an insoluble salt by lowering the concentration of the anion from the solution and forming a weak acid.

AgCl ↔ Ag+ + Cl ?

• What happens to this equilibrium if ammonia or a strong base is added?

Adding Strong Bases or Ammonia to Insoluble Salts

• A strong base or ammonia will often dissolve an insoluble salt by lowering the concentration of the cation from the solution and forming a complex ion.

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