Solubility Simultaneous Equilibria Part I Ksp and Solubility

Solubility & Simultaneous Equilibria Part I: Ksp and Solubility Jespersen Chap. 18 Sec 1 Dr. C. Yau Spring 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) Na 2 S b) Mg. Br 2 c) Pb. Br 2 d) Sr. SO 4 e) CH 3 CH 2 CH 3 Will a precipitate form when you mix aq solns of NH 4 Br with Ba(NO 3)2? 2

Memorize these Solubility Rules *Metal oxides react with water to form hydroxides: e. g. Na 2 O (s) + H OH (l) 2 Na. OH (aq) **NH 4 OH (aq) is soluble but most of it converts to NH 3 (g) + H 2 O (l) 3

Solubility Product Constant (Ksp) Consider the insoluble salt, Ag. Cl. Even though it is said to be "insoluble" it does dissolve to some extent. Ag. Cl (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? Ag. Cl (s) Ag+ (aq) + Cl- (aq) Equilibrium is reached only if the solid and dissolved ions are present. Imagine adding very small amounts of solid Ag. Cl to a beaker of water. At first all of the solid Ag. Cl will dissolve. [Ag+] and [Cl-] will increase as more Ag. Cl(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 5 more solid Ag. Cl 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? (a) calcium oxalate (b) silver sulfate 6

What does Q tell us? Ag. Cl (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. 7

Which is more soluble? Solubility of Ag. Br is 6. 9 x 10 -7 mol L-1 and that of Ag 2 Cr. O 4 is 6. 7 x 10 -5 mol L-1. Which is more soluble? Ans. Ag 2 Cr. O 4 Predict which has the larger Ksp. That of Ag 2 Cr. O 4? (Calculations on next slide) 8

Example 18. 1 p. 833 Silver bromide is the light-sensitive cmpd used in nearly all photographic film. The solubility of Ag. Br in water was measured to be 1. 3 x 10 -4 g L-1 at 25 o. C. Calculate Ksp for Ag. Br at that temperature. Example 18. 2 p. 835 The molar solubility of silver chromate in water is 6. 7 x 10 -5 mol L-1 at 25 o. C. What is Ksp for Ag 2 Cr. O 4? Compare the Ksp of the Ag. Br with that of Ag 2 Cr. O 4. …but Ag 2 Cr. O 4 is more soluble! Why? Do Pract Exer 3, 4 & 5 p. 836 9

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: Ag. Cl compared with Ba. SO 4 (1 cation: 1 anion) (even if the charges of the ions are different: Ag+Br - and Ba 2+SO 42 -) not when ratios are different: Ag. Cl compared with Ag 2 Cr. O 4 1: 1 2: 1 10

Ksp of Ag. Cl vs. Ag 2 Cr. O 4 Ksp (Ag. Cl) = 1. 8 x 10 -10 Ksp (Ag 2 Cr. O 4) = 2. 6 x 10 -12 More soluble? For Ag. Cl, Ksp = 1. 8 x 10 -10 = [Ag+][Cl-] x = 1. 3 x 10 -5 M = x 2 For Ag 2 Cr. O 4, Ksp = 2. 6 x 10 -12 = [Ag+]2[Cr. O 42 -] = (2 x)2 (x) x = 8. 7 x 10 -5 M = 4 x 3 Ag. Cr. O 4 slightly more soluble Solubility should be based on molar solubility rather than Ksp. However, often, Ksp is a good indication of solubility. 11

Example 18. 3 p. 837 What is the molar solubility of Ag. Cl in pure water? You look up Ksp for Ag. Cl (1. 8 x 10 -10) Example 18. 4 p. 837 Calculate the molar solubility of lead(II) iodide in water from its Ksp at 25 o. C. Ksp = 7. 9 x 10 -9 Do Pract Exer 6 & 7 on p. 838 12

The Common Ion Effect Example 18. 5 p. 839 What is the molar solubility of Pb. I 2 in a 0. 10 M Na. I solution? Look up Ksp from Table 18. 1 p. 832 Ksp = 7. 9 x 10 -9 Do Pract Exer 8 & 9 p. 840 13

Predicting Precipitation Example 18. 7 p. 841 Suppose we wish to prepare 0. 500 L of a soln containing 0. 0075 mol of Na. Cl and 0. 075 mol of Pb(NO 3)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 Pb. Cl 2 is 1. 7 x 10 -5. 14 Pract Exer 12 & 13 p. 843

Example 18. 7 p. 842 What possible ppt might form by mixing 50. 0 m. L of 1. 0 x 10 -4 M Na. Cl with 50. 0 m. L of 1. 0 x 10 -6 M Ag. NO 3? Write the net ionic equation. Will the ppt form? Ksp from Table 18. 1 p. 832: Ksp = 1. 8 x 10 -10 Do Pract Exer 12, 13, p. 843. We are skipping Section 18. 2. 15