Chemistry 1011

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Chemistry 1011

• TOPIC
• Solubility Equilibrium
• TEXT REFERENCE
• Masterton and Hurley Chapter 16.1

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16.1 Solubility Equilibrium

• YOU ARE EXPECTED TO BE ABLE TO
• Write an expression for the solubility product
  constant, Ksp, for a substance
• Calculate the concentration of ions at
  equilibrium, given Ksp
• Calculate the solubility product constant for a
  substance given its solubility and formula
• Predict whether a combination of ions will form a
  precipitate, given Ksp and ion concentrations
• Calculate the solubility of a substance in water,
  given Ksp
• Use Le Chateliers Principle to determine the
  effect of adding a common ion to a solution.
• Calculate the solubility of a substance in the
  presence of a common ion

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Formation of Precipitates

• A precipitate is formed when
• Two solutions are mixed, and
• The cation from one solution combines with the
  anion from the other solution to form an
  insoluble solid
• NaCl(aq) AgNO3(aq) NaNO3(aq)
  AgCl(s)
• Na(aq) Cl-(aq) Ag(aq) NO3-(aq)
  Na(aq) NO3-aq) AgCl(s)
• Ag(aq) Cl-(aq) AgCl(s)
• An equilibrium is established between the solid
  and the corresponding ions in solution

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Solubility Equilibrium

• AgCl(s) Ag(aq) Cl-(aq)
• An expression can be written for the equilibrium
  constant
• Ksp AgxCl-
• solid does not appear in the equilibrium
  constant expression
• Ksp is known as the solubility product constant
• Solubility product data are normally measured at
  25oC

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Determining Ion Concentrations

• Ag3PO4(s) 3Ag(aq) PO43-(aq)
• Ksp Ag3xPO43- 1 x 10-16
• PbCl2(s) Pb2(aq) 2Cl-(aq)
• Ksp Pb2xCl-2 1.7 x 10-5
• Q Calculate Pb2 and Cl- in a solution of
  PbCl2 at 25oC
• Cl- 2 x Pb2
• Ksp Pb2 x 2Pb22 1.7 x 10-5
• Ksp 4 Pb23 1.7 x 10-5
• Pb2 1.6 x 10-2 mol/L Cl- 3.2 x
  10-2 mol/L

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Calculating Ksp

• The solubility of a salt can be determined by
  experiment
• Ksp for the salt can be determined from these
  results
• Q The solubility of magnesium hydroxide is found
  to be 8.4 x 10-4 g/100cm3 at 25oC. Find Ksp
• Mg(OH)2(s) Mg2(aq) 2OH-(aq)
• Ksp Mg2xOH-2 ??
• Solubility 8.4 x 10-4 g/100cm3 at 18oC
• Solubility (8.4 x 10-4)g x 1000cm3/L 1.44 x
  10-4 mol/L
• 58.3 g/mol 100cm3
• Mg2 1.44 x 10-4 mol/L OH- 2.88 x 10-4
  mol/L
• Ksp Mg2xOH-2 1.2 x 10-11

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Determining Precipitate Formation

• To determine whether a precipitate will form when
  two solutions are mixed
• Determine the concentrations of the reacting ions
  in the mixture
• Calculate the ion product, P
• Compare the ion product, P, with Ksp
• If P gt Ksp then precipitate will form
• If P lt Ksp then no precipitate
• If P then no precipitate solution is
  saturated

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Determining Precipitate Formation

• Q Will a precipitate form when 5.0mL of 1.0 x
  10-3 mol/L silver nitrate is added to 5.0mL of
  1.0 x 10-5 mol/L potassium chromate? Ksp
  Ag2CrO4 1.0 x 10-12
• 2AgNO3(aq) K2CrO4(aq) 2KNO3(aq)
  Ag2CrO4(s)
• 2Ag(aq) CrO42-(aq) Ag2CrO4(s)
• Ksp Ag2 x CrO42- 1.0 x 10-12
• Ag 5.0 x 10-4 mol/L
• CrO42- 5.0 x 10-6 mol/L
• Ion Product, P Ag2 x CrO42- (5.0 x 10-4
  )2 x (5.0 x 10-6 )
• P 1.25 x 10-12
• P gt Ksp A precipitate will form

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Determining Precipitate Formation

• Q 1.0 mL of 1.0 mol/L barium chloride is added
  to 10.0 mL of a solution containing a small
  amount of magnesium sulfate. Determine the
  minimum concentration of magnesium sulfate that
  will cause a precipitate to form.
• Ksp BaSO4 1.1 x 10-10

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Determining Precipitate Formation

• 1.0 mL of 1.0 mol/L barium chloride is added to
  10.0 mL of a solution containing a small amount
  of magnesium sulfate. Determine the minimum
  concentration of magnesium sulfate that will
  cause a precipitate to form. Ksp BaSO4 1.1
  x 10-10
• BaCl2(aq) MgSO4(aq) MgCl2(aq)
  BaSO4(s)
• Ba2(aq) SO42-(aq) BaSO4(s)
• Ksp Ba2xSO42- 1.1 x 10-10
• Ba2 1.0 x 10-3L x 1.0 mol/L ? 1.00 x 10-2L
  1.0 x 10-1 mol/L
• SO42- x mol/L
• Ksp Ba2xSO42- (1.0 x 10-1 ) x (x) 1.1
  x 10-10
• x SO42- minimum MgSO4 1.1 x 10-10 mol/L

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Selective Precipitation

• Suppose that a solution contains two different
  cations, for example Ba2 and Ca2
• Each forms an insoluble sulfate, BaSO4 and CaSO4
• Ksp BaSO4 1.1 x 10-10
• Ksp CaSO4 7.1 x 10-5
• If sulfate ions are added to a solution
  containing equal amounts of Ba2 and Ca2, then
  the BaSO4 will precipitate first
• Only when the Ba2 ion concentration becomes very
  small will the SO42- ion concentration rise to
  the point that CaSO4 will be precipitated

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Determining Solubility

• The solubility, s, of a salt can be determined
  from Ksp data
• Q Determine the solubility of lead chloride in
  water at 25oC. Ksp 1.7 x 10-5
• Let solubility of lead chloride s mol/L
• For every mole of PbCl2 that dissolves,
• 1 mole of Pb2(aq) and 2 moles of Cl-(aq)
  are formed
• PbCl2(s) Pb2(aq) 2Cl-(aq)
• Pb2 s mol/L
• Cl- 2 x Pb2 2s mol/L
• Ksp Pb2xCl-2 (s) x(2s)2 1.7 x 10-5
• 4s3 1.7 x 10-5
• s 1.6 x 10-2 mol/L (Can also be expressed in
  grams/Litre)

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The Common Ion Effect

• The presence of a common ion will reduce the
  solubility of an ionic salt (Le Chatelier)
• If a common ion is added to a saturated solution
  of a salt, then the salt will be precipitated (Le
  Chatelier)
• For example, CaCO3 is less soluble in a
  solution containing CO32- ions than in pure water
• CaCO3(s) Ca2(aq) CO32-(aq)
• Ksp Ca2 x CO32- 4.9 x 10-9

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The Common Ion Effect

• CaCO3(s) Ca2(aq) CO32-(aq)
• Ksp Ca2 x
  CO32- 4.9 x 10-9
• Solubility of CaCO3 Ca2
• In pure water
• Ca2 CO32- ?(4.9 x 10-9)
  7.0 x 10-5 mol/L
• Solubility of CaCO3 in 1.0 x 10-1 sodium
  carbonate solution ???
• CO32- 1.0 x 10-1 mol/L (ignore CO32- from
  CaCO3 )
• Ca2 Ksp 4.9 x 10-9
  4.9 x 10-8 mol/L
• CO32- 1.0 x
  10-1

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One More Example

• Ksp for manganese II hydroxide is 1.2 x 10-11
• Solid sodium hydroxide is added slowly to a 0.10
  mol/L solution of manganese II chloride. What
  will be the pH when a precipitate forms?
• Mn(OH)2(s) Mn2(aq) 2OH-(aq)
• Ksp Mn2 x OH-2
  1.2 x 10-11
• Mn2 0.10 mol/L
• OH- ? Ksp ? Mn2 ? 1.2 x 10-11 ? 0.10
• 1.1 x 10-5 mol/L
• pH 9.0