Chapter 18 The Solubility Product Constant

Review Quiz Nuclear Chemistry Thermochemistry –Hess’s Law –Heats (Enthalpies) of…

Write the net ionic equation Solutions of sodium chromate and barium chloride are mixed.

Ba 2+ + CrO 4 2- → BaCrO 4

Ba 2+ + CrO 4 2- ↔ BaCrO 4

BaCrO 4 ↔ Ba 2+ + CrO 4 2- Since the reaction is reversible when can flip it. Write the equilibrium expression for this reaction.

BaCrO 4 ↔ Ba 2+ + CrO 4 2- K = [Ba 2+ ] [CrO 4 2- ]

BaCrO 4 ↔ Ba 2+ + CrO 4 2- K sp = [Ba 2+ ] [CrO 4 2- ] 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 K sp = [Ag + ] [Cl - ]

Write the solubility product expression for MgF 2 K sp = [Mg 2+ ] [F - ] 2

Write the solubility product expression for Ag 3 PO 4 K sp = [Ag + ] 3 [PO 4 3- ]

Molar Solubility Molar solubility is the maximum solubility of a substance expressed in moles per liter. Example: The molar solubility of silver chromate (Ag 2 CrO 4 ) is 1.3 x mol/L

The molar solubility of Hg 2 Cl 2 is 6.5 x mol/L. Find the solubility product. K sp = 1.1 x

The molar solubility of Ag 2 CrO 4 is 1.3 x mol/L. Find the solubility product. K sp = 8.8 x

The solubility of PbCrO 4 is 4.30 x g/L. Find the solubility product. What’s the difference here?

The solubility of PbCrO 4 is 4.30 x g/L. Find the solubility product. K sp = 1.8 x

The solubility of PbCrO 4 is 4.30 x g/L. Find the solubility product. K sp = 1.8 x See Appendix D (Pages A6 & A7)

What is the molar solubility PbCrO 4 ? K sp of PbCrO 4 = 1.8 x Molar solubility = 1.3 x mol PbCrO 4 /L [Pb 2+ ] = [CrO 4 2- ] = 1.3 x M What are the ion concentrations?

What is the molar solubility Ag 2 CrO 4 ? K sp of Ag 2 CrO 4 = 8.8 x Molar solubility = 1.3 x mol Ag 2 CrO 4 /L [Ag + ] = 2.6 x M [CrO 4 2- ] = 1.3 x M What are the ion concentrations?

What is the molar solubility Ag 3 PO 4 ? K sp of Ag 3 PO 4 = 1.8 x Molar solubility = 1.6 x mol Ag 3 PO 4 /L [Ag + ] = 4.8 x M [PO 4 3- ] = 1.6 x M What are the ion concentrations?

What is the molar solubility Pb 3 (PO 4 ) 2 ? K sp of Pb 3 (PO 4 ) 2 = 3.0 x Molar solubility = 7.7 x mol Pb 3 (PO 4 )/L [Pb 2+ ] = 2.3 x M [PO 4 3- ] = 1.5 x M What are the ion concentrations?

I place a large scoopful of Ag 3 PO 4 (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 Ag 3 PO 4 dissolve? What are the concentrations in the solution?

What is the molar solubility Ag 3 PO 4 ? K sp of Ag 3 PO 4 = 1.8 x  Molar solubility = 1.6 x mol Ag 3 PO 4 /L  [Ag + ] = 4.8 x mol Ag + /L  [PO 4 3- ] = 1.6 x mol PO 4 3- /L What are the ion concentrations?

Did any of the Ag 3 PO 4 dissolve? If any does dissolve what are the concentrations in the solution?  [Ag + ] = 4.8 x M  [PO 4 3- ] = 1.6 x M

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

What would we observe if we added sodium phosphate solution to a beaker saturated solution of Ag 3 PO 4 ?

What would we observe if we added sodium phosphate solution to the beaker containing a saturated solution of Ag 3 PO 4 ? K sp of Ag 3 PO 4 = 1.8 x [Ag + ] = 4.8 x mol Ag + /L [PO 4 3- ] = 1.6 x mol PO 4 3- /L [Ag + ] 3 [PO 4 3- ] = [4.8 x ] 3 [1.6 x ] = ? Why does Ag 3 PO 4 precipitate?

Reaction Quotient If Q = K sp, the system is at equilibrium and the solution is saturated If Q < K sp, the solution is not saturated precipitate does not form If Q > K sp, the solution exceeds saturation and a precipitate forms Q vs K sp

Does AgCl precipitate when equal volumes of 1.5 x M solutions of AgNO 3 and NaCl mix? ? ?

Does AgCl precipitate when equal volumes of 1.5 x M solutions of AgNO 3 and NaCl mix? NO

Does AgCl precipitate when equal volumes of 1.5 x M solutions of AgNO 3 and NaCl mix?

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

Yes

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

AgNO 3 solution is added to a solution of Na 2 CrO 4. What precipitate forms?

AgNO 3 is added to a M solution of Na 2 CrO 4. What is the [Ag + ] when the Ag 2 CrO 4 precipitate begins to form?

Clothing washed in water that has a manganese concentration exceeding 0.1 mg L -1 (1.8 x M) may be stained with manganese. A laundry wishes to add a base to precipitate manganese as the hydroxide Mn(OH) 2 (K sp = 4.5 x ). At what pH is [Mn 2+ ] equal to 1.8 x 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 (K sp values) in order to form a precipitate. The ion with the ________ K sp will precipitate first. smaller

A solution contains mol of KI and 0.10 mol of KCl per liter. AgNO 3 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 mol of KI and 0.10 mol of KCl per liter. AgNO 3 is gradually added to this solution. Which precipitate forms first, AgCl or AgI?

A solution contains mol of KI and 0.10 mol of KCl per liter. AgNO 3 is gradually added to this solution. What is the [I ̄ ] in the solution when AgCl starts to precipitate?

A solution contains mol of KI and 0.10 mol of KCl per liter. AgNO 3 is gradually added to this solution. What percent of the original [I ̄ ] remains in solution when AgCl starts to precipitate?

NH 4 Cl is added to a 750mL solution that is 0.10M in Mg 2+ and 0.10M in NH 3. The solution also contains a Mg(OH) 2 precipitate. Calculate the [NH 4 + ] that is required to dissolve the precipitate.

NH 4 Cl is added to a 750mL solution that is 0.10M in Mg 2+ and 0.10M in NH 3. The solution also contains a Mg(OH) 2 precipitate. How many grams of NH 4 Cl 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 (CaSO 4 ) is “insoluble” (slightly soluble) in water. Write the equation that represents the solubility of calcium sulfate in water. CaSO 4 (s) ⇌ Ca 2+ (aq) + SO 4 2− (aq)

The Common-Ion Effect CaSO 4 (s) ⇌ Ca 2+ (aq)+SO 4 2− (aq) Describe the equilibrium system of Calcium Sulfate in water. The solution is saturated with most of the calcium sulfate in solid form. The [SO 4 2− ] in the solution is very low and is equal to the [Ca 2+ ].

The Common-Ion Effect CaSO 4 (s) ⇌ Ca 2+ (aq)+SO 4 2− (aq) Describe what would happen if a solution of sodium sulfate were added to the beaker. The [SO 4 2− ] would increase causing the reaction to shift to the left reducing the solubility of CaSO 4 causing the it CaSO 4 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 ↔ Al OH ̄ 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 –, CH 3 CO 2 –, or CN – it is normally more soluble at low pH’s. –When a salt contains an acidic ion such as Ag +, Zn 2+, or Al 3+ it is often normally more soluble at high pH’s.

FeS ↔ Fe 2+ + S 2- 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.

Hidden Slide Remaining slides are hidden

Demo the previous slide Form AgCl precipitate by adding solutions of NaCl and AgNO 3. Have concentrated solutions of NaOH and NH 3 available. Have students chose which they would like to see added to dissolve the precipitate.

Calculate the pH required to prevent the precipitation of ZnS in a M ZnCl 2 solution that is saturated with H 2 S (0.10M H 2 S).