The Solubility Product Constant Ksp

Ksp: review (grade 11) 1) What is the molar mass of H2O? 2) How many moles are in 18 g of NaCl?

3) How many grams of CaCl2 are found in 2 L of a 3 M solution of CaCl2? 4) What is the concentration (in mol/L) of K+ when 2 L of 1.5 M KCl is mixed with 1 L of 3 M K2SO4?

Ksp (solubility product) - background Ksp is similar to K, It deals with ions instead of gases, One side of the chem. equation has a solid, which is ignored There are other types of K: Ksp, Kw, Ka, Kb Each subscript immediately indicates some detail about the equilibrium Kw: equilibrium of water, Ka: acid, Kb: base Ksp: equilibrium between solid and ions

Ksp deals with a phase equilibrium: (s)  (aq) Ksp - background The equilibrium between solids and ions is different from the equilibrium between gases The equilibrium between solids and ions is a “phase” equilibrium (e.g. NaCl(aq)) NaCl(s) Na+(aq) + Cl-(aq) Ksp deals with a phase equilibrium: (s)  (aq)

Ksp - Molar Solubility A solid always dissolves until no more can dissolve - called molar solubility (mol/L or M) Ksp and molar solubility describe the solubility of a substance An equilibrium is established when the amount dissolving equals the amount precipitating This can only be true if there is some solid (thus, we can usually see if there is an equilibrium) A solution with solid remaining (I.e. in equilibrium) is called “saturated” Note: adding more solid will not affect equilibrium

Ex. 1 Calculate the molar solubility of zinc hydroxide at 25°C, where Ksp is 7.7×10-17

Predicting Precipitation solubility—to predict whether a precipitate will form when we mix solutions of metal cations and nonmetal anions. sometimes called the trial ion product previously, you used solubility tables to examine solubility To predict whether a precipitate will form when solutions containing anions and cations are mixed, we compare the Ksp value for the salt of these ions Q>Ksp, precipitation occurs “supersaturated Q<Ksp,” no precipitation occurs Q=Ksp, no precipitation occurs, system @ Eq’m

Ex. 2: If 100 mL of 0. 100 mol/L CaCl2(aq) and 100 mL of 0 Ex. 2: If 100 mL of 0.100 mol/L CaCl2(aq) and 100 mL of 0.0400 mol/L Na2SO4 (aq) are mixed at 20°C, determine whether a precipitate will form. For CaSO4 (aq) at 20°C, Ksp is 3.6 ×10–5.

The Common Ion Effect Common ion: “The ion in a mixture of ionic substances that is common to the formulas of at least two.” Common ion effect: “The solubility of one salt is reduced by the presence of another having a common ion”

Ex: 3 What is the molar solubility of lead in PbI2 in a 0 Ex: 3 What is the molar solubility of lead in PbI2 in a 0.10mol/L sodium iodide solution. For PbI2 at SATP Ksp = 7.9 x 10–9