Chapter 21 Notes, part III Ksp and Common Ion Effect

Solubility Product Constant Salts have different solubilities; even salts we earlier considered insoluble have a very, very small amount of solubility in water.Salts have different solubilities; even salts we earlier considered insoluble have a very, very small amount of solubility in water. The solubility product constant (K sp ) is an index of how soluble a salt is— the bigger the Ksp, the more soluble the salt is.The solubility product constant (K sp ) is an index of how soluble a salt is— the bigger the Ksp, the more soluble the salt is.

K sp K sp is based on K eq from Ch. 19; for example:K sp is based on K eq from Ch. 19; for example: K eq for AgCl(s)  Ag + (aq) + Cl - (aq) would be:K eq for AgCl(s)  Ag + (aq) + Cl - (aq) would be: K eq = But a solid is not dissolved, and thus does not have a constant, so:But a solid is not dissolved, and thus does not have a constant, so: K sp = [Ag + ][Cl - ] [Ag + ][Cl - ] [AgCl]

Practice Problem #1 What is the Ksp for Ag 2 SO 4 if its solubility is 3.5x M?What is the Ksp for Ag 2 SO 4 if its solubility is 3.5x M?

Practice Problem #2 What is the concentration of silver ions and chlorine ions in AgCl if K sp =1.8x at 25 o C?What is the concentration of silver ions and chlorine ions in AgCl if K sp =1.8x at 25 o C?

Practice Problem #3 Calcium fluoride has K sp =3.9x What is the concentration of fluoride ions?Calcium fluoride has K sp =3.9x What is the concentration of fluoride ions?

Practice Problem #4 K sp for Ag 2 S is 8x What is the concentration of Ag + ions?K sp for Ag 2 S is 8x What is the concentration of Ag + ions?

Common Ion Effect A common ion is an ion that two salts or a salt and an acid or base have in common.A common ion is an ion that two salts or a salt and an acid or base have in common. The lowering of solubility of a substance by the addition of a common ion is called the common ion effect.The lowering of solubility of a substance by the addition of a common ion is called the common ion effect.

Common Ion Effect For example:For example: PbCrO 4  Pb 2+ + CrO 4 2- Ksp=1.8x10 -14PbCrO 4  Pb 2+ + CrO 4 2- Ksp=1.8x If we add lead (II) nitrate (Pb(NO 3 ) 2 ), we are adding a common ion (see the dissociation reaction on the board).If we add lead (II) nitrate (Pb(NO 3 ) 2 ), we are adding a common ion (see the dissociation reaction on the board).

Common Ion Effect Due to LeChatelier’s principle, the reaction will shift left because of the increased Pb 2+ concentration, causing formation of PbCrO 4 solid.Due to LeChatelier’s principle, the reaction will shift left because of the increased Pb 2+ concentration, causing formation of PbCrO 4 solid.

Practice Problem #5 Predict whether barium sulfate will precipitate if of mol barium nitrate is mixed with 1.0L of M sodium sulfate to form a 1.0L solution (Ksp of BaSO 4 = 1.1x )Predict whether barium sulfate will precipitate if of mol barium nitrate is mixed with 1.0L of M sodium sulfate to form a 1.0L solution (Ksp of BaSO 4 = 1.1x )

Practice Problem #6 Predict whether silver cyanide will precipitate if 500mL of M silver nitrate is mixed with 500mL of M sodium cyanide to form a 1.0L solution (Ksp of AgCN=2.0x )Predict whether silver cyanide will precipitate if 500mL of M silver nitrate is mixed with 500mL of M sodium cyanide to form a 1.0L solution (Ksp of AgCN=2.0x )