Solubility Equilibria

Will it all dissolve, and if not, how much?

All dissolving is an equilibrium.If there is not much solid it will all

dissolve.As more solid is added the solution

will become saturated.Solid dissolvedThe solid will precipitate as fast as

it dissolves .Equilibrium….aka…saturated

solution

Watch outSolubility (molar solubility, s) is

not the same as solubility product.

Solubility product is an equilibrium constant.

it doesn’t change except with temperature.

Solubility is an equilibrium position for how much can dissolve.

A common ion can change this.

Practice: Calculate the Ksp

The solubility of iron(II) oxalate FeC2O4 is 65.9 mg/L

◦Ksp = x2 = 4343

The solubility of Li2CO3 is 5.48

g/L◦Ksp = (2x)2x = 4x3 = 658

Calculating SolubilityThe solubility is determined by

equilibrium.Its an equilibrium problem.Watch the coefficients

Practice: Calculating SolubilityCalculate the solubility of SrSO4,

with a Ksp of 3.2 x 10-7 in M and g/L.◦3.2 x 10-7 = x2

◦x = 5.7 x 10-4

Calculate the solubility of Ag2CrO4, with a Ksp of 9.0 x 10-12

in M and g/L.◦9.0 x 10-12 = (2x)2x = 4x3 ◦x = 1.31 x 10-4

Relative Solubilities: ReminderKsp will only allow us to compare

the solubility of solids that fall apart into the same number of ions.

The bigger the Ksp of those the more soluble.

If they fall apart into different number of pieces you have to do the math.

Common Ion EffectIf we try to dissolve the solid in a

solution with either the cation or anion already present less will dissolve.

Common Ion EffectHow is the solubility of SrSO4, with a

Ksp of 3.2 x 10-7 effected by the addition of a solution of 0.010 M Na2SO4

SrSO4 Sr2+ + SO42-

Na2SO4 2Na+ + SO42-

Common ion decreases the solubility

Common Ion EffectHow is the solubility of SrSO4, with a Ksp of 3.2 x 10-7 effected by the addition of a solution of 0.010 M Sr(NO3)2.

SrSO4 Sr2+ + SO42-

Sr(NO3)2 Sr2+ + 2NO3-

Common ion decreases the solubility

Solubility and pHMg(OH)2 solubility is pH dependent. How?In a BASIC solution, the concentration of

hydroxide ion in solution is high.◦So solubility is LOWER than in pure water.

In an ACIDIC solution, we have a significant amount of hydronium, which can react with hydroxide. This lowers the hydroxide concentration and makes◦So solubility if higher than in pure water

pH and solubilityDissociate the salt to determine if the

solution is acidic, neutral or basic

If a compound is BASIC, then it will be LESS SOLUBLE in basic solutions, and MORE SOLUBLE in acidic solutions!

If a compound is ACIDIC, then it will be MORE SOLUBLE in basic solutions, and LESS SOLUBLE in acidic solutions!

If a compound is NEUTRAL (neither acidic nor basic), then its solubility will be UNAFFECTED by pH

pH and solubility

Na3PO4

Compared to water, how is the solubility effected in an acid? in a base?

Na3PO4 3Na+ + PO43-

PO43- + H2O HPO4

2- + OH-

wa sb

One More Twist

Explain how the solubility of AgCl is affected by adding an ammonia solution.

AgCl + NH3 [Ag(NH3)2]+ + Cl-

Solubility increases as the silver dissolves to form the complex ion.