VII. Precipitation Equilibria
VII-1. Introduction and Background VII-2. Sample Problem VII-3. List of Problems VII-4. Simple Problems VII-5. Advanced Problems

Print this list and try working these problems before checking your answers and looking at the hints.

Simple Problems

• VII-4(1). What are the equilibrium concentrations of Pb2+ and CrO42- when solid PbCrO4 (s) is placed in water?
Unbalanced reaction: PbCrO4 (s) Pb2+(aq) + CrO42-(aq)     Ksp = 1.8x10-14 at 25oC.
• VII-4(2). Calculate Ksp for CaF2 (s) given the following measurements for a water solution that is saturated with solid CaF2:     [Ca2+] = 2.3x10-4 M     [F-] = 4.6x10-4 M.
Unbalanced reaction: CaF2 (s) Ca2+(aq) + F-(aq) (unbalanced).
• VII-4(3). Find the [Ag+] in water and in a solution of 1.0 M NaCl when solid AgCl is added to each solution.
Unbalanced reaction: AgCl(s) Ag+(aq) + Cl-(aq)     Ksp = 1.82x10-10 at 25oC.

• VII-5(1). What is the pH range that would precipitate heavy-metal pollutants such as cadmium, lead, and manganese from an industrial waste stream, but not magnesium and calcium? The heavy metals are present at concentrations of approximately 0.002 M, and the magnesium and calcium concentrations are 0.005 M. The relevant unbalanced reactions and solubility products for these metals are:
Ca(OH)2 (s) Ca2+(aq) + OH-(aq)     Ksp = 1.3x10-6
Cd(OH)2 (s) Cd2+(aq) + OH-(aq)     Ksp = 5.9x10-15
Mg(OH)2 (s) Mg2+(aq) + OH-(aq)     Ksp = 8.9x10-12
Mn(OH)2 (s) Mn2+(aq) + OH-(aq)     Ksp = 1.9x10-13
Pb(OH)2 (s) Pb2+(aq) + OH-(aq)     Ksp = 1.2x10-15
• VII-4(2). Will a precipitate form when 1.0 ppm fluoride is added to drinking water that contains 0.010 M Ca2+? What is the maximum concentration of Ca2+ to which 1.0 ppm fluoride can be added without a precipitate forming?
Unbalance reaction: CaF2 (s) Ca2+(aq) + F-(aq)         Ksp = 4.9x10-11.
• VII-5(3). Will a precipitate form in a sample of rain runoff from an old coal mine if it is neutralized to a pH of 7.0 with 1 M NaOH? Assume that the water sample contains approximately 0.001 M Fe3+ and 0.010 M H2SO4. If a precipitate forms, what is the resulting Fe3+(aq) concentration? What is the Fe3+ concentration if enough NaOH is added to raise the pH of the solution to 10.0?
Unbalance reaction: Fe(OH)3 (s) Fe3+(aq) + OH-(aq)         Ksp = 4x10-38.

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