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AP Chemistry

Unit 7.11

INTRODUCTION TO SOLUBILITY EQUILIBRIA

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Unit 7.10

Enduring Understanding:

  • The dissolution of a salt is a reversible process that can be influenced by environmental factors such as pH or other dissolved ions

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Learning Objective:

  • Calculate the solubility of a salt based on the value of KSP for the salt

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Solubility Product

  • Solubility: the amount of salt that will dissolve to form a saturated solution at a given temperature
    • g/L or mol/L
  • Not all substances are completely soluble or insoluble
    • Some are partially soluble
  • The process of dissolution is reversible
    • The solid and aqueous ions are in dynamic equilibrium
    • Example:
      • PbCl2 (s) ⇌ Pb2+ (aq) + 2Cl- (aq)

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Solubility Product

  • KSP: the equilibrium constant expression for a dissolution reaction
    • “SP” = “solubility product”
  • The KSP represents the maximum amount of ions that can be present in the solution before a precipitate is formed
  • Since solids are not included in K expressions, KSP never has a denominator

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Solubility Product

  • KSP values indicate the extent to which a solid will dissolve
    • Higher values: more dissolution
    • Lower values: less dissolution
    • **See notes on slide 23 for more details on this and how to avoid common misconceptions on application of this
  • KSP is usually only for partially soluble solids
    • These will have reversible reactions
    • Completely soluble substances will be irreversible reactions; their dissolution will go to completion
      • SNAP ions

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Solubility Product

  • Types of questions:
    • Find the solubility of the solid from the KSP value

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Solubility Product

  • Types of questions:
    • Find the solubility of the solid from the KSP value

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Example:

How much PbCl2 can dissolve given KSP = 1.7 x 10-5

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PbCl2 (s) ⇌ Pb2+ (aq) + 2Cl- (aq)

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Solubility Product

  • Types of questions:
    • Find the solubility of the solid from the KSP value

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Example:

How much PbCl2 can dissolve given KSP = 1.7 x 10-5

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PbCl2 (s) ⇌ Pb2+ (aq) + 2Cl- (aq)

KSP = 1.7 x 10-5 = [Pb2+][Cl-]2

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Solubility Product

  • Types of questions:
    • Find the solubility of the solid from the KSP value

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Example:

How much PbCl2 can dissolve given KSP = 1.7 x 10-5

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PbCl2 (s) ⇌ Pb2+ (aq) + 2Cl- (aq)

KSP = 1.7 x 10-5 = [Pb2+][Cl-]2

KSP = 1.7 x 10-5 = (S)(2S)2

Let S represent the amount of PbCl2 that can dissolve (S for solubility)

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Solubility Product

  • Types of questions:
    • Find the solubility of the solid from the KSP value

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Think of this like an ICE table:

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KSP = 1.7 x 10-5 = (S)(2S)2

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PbCl2

Pb2+

2Cl-

I

Some solid; the solution is saturated

0

0

C

+S

+2S

E

S

2S

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Solubility Product

  • Types of questions:
    • Find the solubility of the solid from the KSP value

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Example:

How much PbCl2 can dissolve given KSP = 1.7 x 10-5

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PbCl2 (s) ⇌ Pb2+ (aq) + 2Cl- (aq)

KSP = 1.7 x 10-5 = [Pb2+][Cl-]2

KSP = 1.7 x 10-5 = (S)(2S)2 = 4S3

Let S represent the amount of PbCl2 that can dissolve (S for solubility)

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Solubility Product

  • Types of questions:
    • Find the solubility of the solid from the KSP value

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Example:

How much PbCl2 can dissolve given KSP = 1.7 x 10-5

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PbCl2 (s) ⇌ Pb2+ (aq) + 2Cl- (aq)

KSP = 1.7 x 10-5 = [Pb2+][Cl-]2

KSP = 1.7 x 10-5 = (S)(2S)2 = 4S3 = 0.016 M

Let S represent the amount of PbCl2 that can dissolve (S for solubility)

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Solubility Product

  • Types of questions:
    • Find the solubility of the solid from the KSP value

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Example:

How much PbCl2 can dissolve given KSP = 1.7 x 10-5

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PbCl2 (s) ⇌ Pb2+ (aq) + 2Cl- (aq)

KSP = 1.7 x 10-5 = [Pb2+][Cl-]2

KSP = 1.7 x 10-5 = (S)(2S)2 = 4S3 = 0.016 M

[PbCl2] = 0.016 M [Pb2+] = 0.016 M [Cl-] = 0.032 M

Let S represent the amount of PbCl2 that can dissolve (S for solubility)

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Solubility Product

  • Types of questions:
    • Find the KSP given the solubility

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Example:

The compound AB3 was dissolved in water. The molarity of ion A3+ is found to be 0.00213 M, what is the value for the KSP of AB3?

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Solubility Product

  • Types of questions:
    • Find the KSP given the solubility

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Example:

The compound AB3 was dissolved in water. The molarity of ion A3+ is found to be 0.00213 M, what is the value for the KSP of AB3?

AB3 (s) ⇌ A3+ (aq) + 3B- (aq)

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Solubility Product

  • Types of questions:
    • Find the KSP given the solubility

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Example:

The compound AB3 was dissolved in water. The molarity of ion A3+ is found to be 0.00213 M, what is the value for the KSP of AB3?

AB3 (s) ⇌ A3+ (aq) + 3B- (aq)

KSP = [A3+][B-]3 = [x][3x]3 = (0.00213)(0.00639)3

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Solubility Product

  • Types of questions:
    • Find the KSP given the solubility

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Example:

The compound AB3 was dissolved in water. The molarity of ion A3+ is found to be 0.00213 M, what is the value for the KSP of AB3?

AB3 (s) ⇌ A3+ (aq) + 3B- (aq)

KSP = [A3+][B-]3 = [x][3x]3 = (0.00213)(0.00639)3 = 5.56 x 10-10

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Ion Ratios

ION RATIO

Examples

KSP in terms of molar solubility, “S”

1:1

AgCl and CaSO4

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1:2

Cu(IO3)2

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1:3

Cr(OH)3

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2:3

Mg3(PO4)2

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Ion Ratios

ION RATIO

Examples

KSP in terms of molar solubility, “S”

1:1

AgCl and CaSO4

KSP = S x S = S2

1:2

Cu(IO3)2

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1:3

Cr(OH)3

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2:3

Mg3(PO4)2

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Ion Ratios

ION RATIO

Examples

KSP in terms of molar solubility, “S”

1:1

AgCl and CaSO4

KSP = S x S = S2

1:2

Cu(IO3)2

KSP = S x (2S)2 = 4S3

1:3

Cr(OH)3

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2:3

Mg3(PO4)2

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Ion Ratios

ION RATIO

Examples

KSP in terms of molar solubility, “S”

1:1

AgCl and CaSO4

KSP = S x S = S2

1:2

Cu(IO3)2

KSP = S x (2S)2 = 4S3

1:3

Cr(OH)3

KSP = S x (3S)3 = 27S4

2:3

Mg3(PO4)2

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Ion Ratios

ION RATIO

Examples

KSP in terms of molar solubility, “S”

1:1

AgCl and CaSO4

KSP = S x S = S2

1:2

Cu(IO3)2

KSP = S x (2S)2 = 4S3

1:3

Cr(OH)3

KSP = S x (3S)3 = 27S4

2:3

Mg3(PO4)2

KSP = (3S)3 x (2S)2 = 108S5

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Salt Solubility

  • Soluble salts have a KSP > 1
    • We typically assume these salts are 100% soluble unless a K value is given or requested
  • Because different compounds have different ratios of ions (as we saw in the ion ratio table),we CAN’T COMPARE KSP VALUES TO DETERMINE SOLUBILITY unless they form compounds in the same ratio
    • You’re much better off comparing S (the actual solubility)

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Exam content

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Salt Solubility

  • Example:
    • Which of the following salts has the highest molar solubility at 25℃? Mg(OH)2, AgCl, BaCO3, or La(IO3)3?

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Salt Solubility

  • Example:
    • Which of the following salts has the highest molar solubility at 25℃? Mg(OH)2, AgCl, BaCO3, or La(IO3)3?

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Salt Solubility

  • Example:
    • Which of the following salts has the highest molar solubility at 25℃? Mg(OH)2, AgCl, BaCO3, or La(IO3)3?

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In terms of KSP: BaCO3 > AgCl > Mg(OH)2 > La(IO3)3

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Salt Solubility

  • Example:
    • Which of the following salts has the highest molar solubility at 25℃? Mg(OH)2, AgCl, BaCO3, or La(IO3)3?

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In terms of KSP: BaCO3 > AgCl > Mg(OH)2 > La(IO3)3

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Salt Solubility

  • Example:
    • Which of the following salts has the highest molar solubility at 25℃? Mg(OH)2, AgCl, BaCO3, or La(IO3)3?

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In terms of S: La(IO3)3 > Mg(OH)2 > BaCO3 > AgCl

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QSP

  • Helps us determine if a precipitate will form
  • If QSP < KSP there are not enough ions
    • Solution is unsaturated
    • No precipitate forms
  • If QSP > KSP there are too many ions
    • Too much product
    • Precipitate forms

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Practice: I Do

  1. The compound XBr3 has a KSP value of 4.52 x 10-8 when XBr3 is added to water, what is the concentration of X3+ and Br- ions?

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Practice: I Do

  • The compound XBr3 has a KSP value of 4.52 x 10-8 when XBr3 is added to water, what is the concentration of X3+ and Br- ions?

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XBr3 (s) ⇌ X3+ (aq) + 3Br (aq)

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Practice: I Do

  • The compound XBr3 has a KSP value of 4.52 x 10-8 when XBr3 is added to water, what is the concentration of X3+ and Br- ions?

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XBr3 (s) ⇌ X3+ (aq) + 3Br (aq)

KSP = [X3+][Br]3 = 4.52 x 10-8

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Practice: I Do

  • The compound XBr3 has a KSP value of 4.52 x 10-8 when XBr3 is added to water, what is the concentration of X3+ and Br- ions?

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XBr3 (s) ⇌ X3+ (aq) + 3Br (aq)

KSP = [X3+][Br]3 = 4.52 x 10-8

KSP = [x][3x]3 = 4.52 x 10-8

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Practice: I Do

  • The compound XBr3 has a KSP value of 4.52 x 10-8 when XBr3 is added to water, what is the concentration of X3+ and Br- ions?

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XBr3 (s) ⇌ X3+ (aq) + 3Br (aq)

KSP = [X3+][Br]3 = 4.52 x 10-8

KSP = [x][3x]3 = 4.52 x 10-8

27x4 = 4.52 x 10-8

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Practice: I Do

  • The compound XBr3 has a KSP value of 4.52 x 10-8 when XBr3 is added to water, what is the concentration of X3+ and Br- ions?

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XBr3 (s) ⇌ X3+ (aq) + 3Br (aq)

KSP = [X3+][Br]3 = 4.52 x 10-8

KSP = [x][3x]3 = 4.52 x 10-8

X = 0.0064

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Practice: I Do

  • The compound XBr3 has a KSP value of 4.52 x 10-8 when XBr3 is added to water, what is the concentration of X3+ and Br- ions?

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XBr3 (s) ⇌ X3+ (aq) + 3Br (aq)

KSP = [X3+][Br]3 = 4.52 x 10-8

KSP = [x][3x]3 = 4.52 x 10-8

[x3+] = 0.0064 M

[Br] = 0.019 M