The Solubility Product Constant (Ksp)

The Solubility Product Constant Remember  not all salts are soluble! AgNO3 (aq) + NaCl (aq)  AgCl (s) + NaNO3 (aq) Write a net ionic equation for the above reaction: Write Keq for the above equation What would the dissolving of AgCl(s) look like in an equation? Wait… is this Keq correct? Are solids included in a Keq equation?

The Solubility Product Constant Since some AgCl remains an undissolved solid, the [AgCl] is constant: Keq [AgCl] = [Ag+]x[Cl-] = Ksp Ksp = solubility product constant Measured at 25ºC Ksp for AgCl = 1.8x10-10 Equilibrium occurs only at and past saturation point…so only if there is a undissolved alt mixed with dissolved salt

Write Ksp for each salt: Practice Problems Write Ksp for each salt: PbCl2 and Ag2CrO4

Practice Problems What is the Ag+ concentration in a saturated AgCl solution at 25ºC? ICE chart can be included but usually not necessary since initial concentration of the ions are considered 0. The salt dissolves and produces x amount of ions…0 plus x equals x. (Ksp for AgCl = 1.8x10-10)

Practice Problems Calcium fluoride has a Ksp of 3.9x10-11 at 25ºC. What is the fluoride ion concentration at equilibrium?

Consider the following equation: PbCrO4 Pb+2 + CrO42- Ksp = 1.8x10-14 The Common Ion Effect Consider the following equation: PbCrO4 Pb+2 + CrO42- Ksp = 1.8x10-14 What happens if we add lead (II) nitrate (Pb(NO3)2)? What are we really adding? Think about Le Châtelier's Principle!

The Common Ion Effect PbCrO4 Pb2+ + CrO42- PbCrO4 Pb2+ + CrO42- The Pb2+ will precipitated until the product of Pb2+ and CrO42- is 1.8x10-14 Extra Pb2+ Added The Ksp (just like the Keq) does not change when you add a reactant or product, the reaction shifts to accommodate this addition.

Common Ion: an ion added that is common to both salts The Common Ion Effect Common Ion: an ion added that is common to both salts Pb2+ was common to PbCrO4 and Pb(NO3)2 Adding a common ion causes solubility to decrease Called the common ion effect

Common Ion Problems The Ksp of AgI is 8.3x10-17. What is the iodide-ion concentration of a 1.00-L saturated solution of AgI to which 0.020 mol AgNO3 is added?\ With a common ion, an ICE chart could be useful. What do you know? Ksp = 8.3x10-17 Mol AgNO3 added = 0.020 AgI  Ag+ + I- Ksp = [Ag+] x [I-] What do you want to know? [I-] = ?M At equilibrium, [Ag+] = 0.020 M Plug that into Ksp to solve for [I-] 8.3x10-17 = (0.020 M)(x) x = 4.2x10-15 Does this make sense? Yes! With [Ag+] being so large, the [I-] should be small!

Another problem… Would precipitation occur when 500mL of a 0.02M solution of AgNO3 is mixed with 500mL of a 0.001M solution of NaCl? (Ksp for AgCl = 1.8x10-10)

Before you go… A student prepares a solution by combining 0.025mol calcium chloride and 0.015mol lead (II) nitrate and adding water to make 1.0L of solution. Will a precipitate of lead (II) chloride form in this solution. Ksp = 1.7x10-5