Solubility & Simultaneous Equilibria Part I: Ksp and Solubility Jespersen Chap. 18 Sec 1 Dr. C. Yau Fall 2014 1

Review Solubility Rules from Gen Chem I Re-Memorize the solubility rules! (see next slide) Which of the following are soluble in water? a) Na2S b) MgBr2 c) PbBr2 d) SrSO4 e) CH3CH2CH3 Will a precipitate form when you mix aq solns of NH4Br with Ba(NO3)2?

Memorize these Solubility Rules *Metal oxides react with water to form hydroxides: e.g. Na2O (s) + H OH (l)  2 NaOH (aq) **NH4OH (aq) is soluble but most of it converts to NH3 (g) + H2O (l)

Solubility Product Constant (Ksp) Consider the insoluble salt, AgCl. Even though it is said to be "insoluble" it does dissolve to some extent. AgCl (s) Ag+ (aq) + Cl- (aq) The equilibrium law is written as Ksp = [Ag+][Cl-] for a saturated solution Q (reaction quotient) is often called the "ion product." If Q < Ksp, the solution is unsaturated. 4

Why is Ksp for Saturated Solns? AgCl (s) Ag+ (aq) + Cl- (aq) Equilibrium is reached only if the solid and dissolved ions are present. Imagine adding very small amounts of solid AgCl to a beaker of water. At first all of the solid AgCl will dissolve. [Ag+] and [Cl-] will increase as more AgCl(s) is added. When no more will dissolve, the solution is said to be saturated. [Ag+] and [Cl-] in the soln cannot be any higher even if more solid AgCl is added.

Practice Exercise 18.1, p. 833 Write the eqn for the equilibrium involved in the solubility of barium phosphate and write the equilibrium law corresponding to Ksp. Practice Exercise 18.2, p. 833 What are the ion product expressions for the following salts? calcium oxalate silver sulfate

What does Q tell us? AgCl (s) Ag+ (aq) + Cl- (aq) Remember the difference between Q and K. You plug in equilibrium concentrations to calc K, and plug in initial concentrations to calc Q. If Q < Ksp, the conc of the ions are less than conc at saturation point, so no precipitate will form. If Q = Ksp, the conc of the ions are at a maximum, no precipitate will form. If Q > Ksp, saturation point has been surpassed. Precipitation will occur.

Which is more soluble? Solubility of AgBr is 6.9x10-7 mol L-1 and that of Ag2CrO4 is 6.7x10-5 mol L-1. Which is more soluble? Ans. Ag2CrO4 Predict which has the larger Ksp. That of Ag2CrO4? (Calculations on next slide)

Example 18.1 p. 833 Silver bromide is the light-sensitive cmpd used in nearly all photographic film. The solubility of AgBr in water was measured to be 1.3x10-4 g L-1 at 25oC. Calculate Ksp for AgBr at that temperature. Example 18.2 p. 835 The molar solubility of silver chromate in water is 6.7x10-5 mol L-1 at 25 oC. What is Ksp for Ag2CrO4? Compare the Ksp of the AgBr with that of Ag2CrO4. …but Ag2CrO4 is more soluble! Why? Do Pract Exer 3, 4 & 5 p. 836

Which is more soluble? Be careful when comparing Ksp to predict solubility. You use Ksp to predict solubility only if the ratios of # cations to # anions are the same: AgCl compared with BaSO4 (1 cation:1 anion) (even if the charges of the ions are different: Ag+Br - and Ba2+SO42-) not when ratios are different: AgCl compared with Ag2CrO4 1: 1 2 : 1

Ksp of AgCl vs. Ag2CrO4 Ksp (AgCl) = 1.8x10-10 Ksp (Ag2CrO4) = 2.6x10-12 More soluble? For AgCl, Ksp = 1.8x10-10 = [Ag+][Cl-] = x2 For Ag2CrO4, Ksp = 2.6x10-12 = [Ag+]2[CrO42-] = (2x)2 (x) = 4x3 x = 1.3x10-5 M x = 8.7x10-5 M AgCrO4 slightly more soluble Solubility should be based on molar solubility rather than Ksp. However, often, Ksp is a good indication of solubility.

Example 18.3 p. 837 What is the molar solubility of AgCl in pure water? You look up Ksp for AgCl (1.8x10-10) Example 18.4 p. 837 Calculate the molar solubility of lead(II) iodide in water from its Ksp at 25oC. Ksp = 7.9x10-9 Do Pract Exer 6 & 7 on p. 838

The Common Ion Effect Example 18.5 p. 839 What is the molar solubility of PbI2 in a 0.10 M NaI solution? Look up Ksp from Table 18.1 p. 832 Ksp = 7.9x10-9 Do Pract Exer 8 & 9 p. 840

Predicting Precipitation Example 18.7 p. 841 Suppose we wish to prepare 0.500 L of a soln containing 0.0075 mol of NaCl and 0.075 mol of Pb(NO3)2. What do the solubility rules tell you? Write the molecular, total ionic and net ionic equation. Will a precipitate form? (Are the concentrations of the ions enough for a ppt to form? Ksp of PbCl2 is 1.7x10-5. Pract Exer 12 & 13 p. 843

Example 18.7 p. 842 What possible ppt might form by mixing 50.0 mL of 1.0x10-4 M NaCl with 50.0 mL of 1.0 x 10-6 M AgNO3? Write the net ionic equation. Will the ppt form? Ksp from Table 18.1 p. 832: Ksp = 1.8x10-10 Do Pract Exer 12, 13, p. 843. We are skipping Section 18.2.