Solubility equilibria
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Solubility Equilibria. Will it all dissolve, and if not, how much will?. SOLUBILITY EQUILIBRIA. Solubility : Relative term used to describe how much of a particular substance dissolves in a certain amount of solvent. Substances that dissolve very well are said to be soluble

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Solubility Equilibria

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Solubility Equilibria

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


  • Solubility: Relative term used to describe how

    much of a particular substance dissolves in a

    certain amount of solvent.

  • Substances that dissolve very well are said to

    be soluble

  • Insoluble species don’t dissolve well.

  • All substances are “soluble” to some extent

  • We will look at slightly soluble substances


  • 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 ↔ dissolved

  • The solid will precipitate as fast as it dissolves, forming an equilibrium.

Watch out

  • Solubility 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.

Ksp Values for Some Salts at25C


  • Consider the following reaction

  • The equilibrium constant expression is

    Ksp = [Pb2+][Cl-]2

  • Ksp is called the solubility product constant or

    simply solubility product

  • For a compound of general formula, MyXz (next page)

Ksp = [Mz+]y[Xy-]z

Ksp = [Mg2+][NH4+][PO43-]

Ksp = [Zn2+][OH-]2

Ksp = [Ca2+]3[PO43-]2

  • Molar solubility: the number of moles that

    dissolve to give 1 liter of saturated solution

  • As with any equilibrium constant the numerical

    value must be determined from experiment

  • The Ksp expression is useful because it applies

    to all saturated solutions

    - the origins of the ions are not relevant

  • Consider that @ 25C Ksp AgI = 1.5 x 10-16

Solving Solubility Problems

For the salt AgI at 25C, Ksp = 1.5 x 10-16

AgI(s)  Ag+(aq) + I-(aq)







1.5 x 10-16 = x2

x = solubility of AgI in mol/L = 1.2 x 10-8 M

Solving Solubility Problems

For the salt PbCl2 at 25C, Ksp = 1.6 x 10-5

PbCl2(s)  Pb2+(aq) + 2Cl-(aq)







1.6 x 10-5 = (x)(2x)2 = 4x3

x = solubility of PbCl2 in mol/L = 1.6 x 10-2 M

Relative Solubilities

  • Ksp will only allow us to compare the solubility of solids the 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.

The Common Ion Effect

  • When the salt with the anion of a weak acid is added to that acid:

    • it reverses the dissociation of the acid.

    • lowers the percent dissociation of the acid.

  • The same principle applies to salts with the cation of a weak base..

  • The calculations are the same as with acid base equilibrium.

Solving Solubility with a Common Ion

For the salt AgI at 25C, Ksp = 1.5 x 10-16

What is its solubility in 0.05 M NaI?

AgI(s)  Ag+(aq) + I-(aq)







1.5 x 10-16 = (x)(0.05+x)  (x)(0.05)

x = solubility of AgI in mol/L = 3.0 x 10-15 M

pH and solubility

  • OH- can be a common ion.

  • More soluble in acid.

  • For other anions if they come from a weak acid they are more soluble in a acidic solution than in water.

  • CaC2O4↔Ca+2 + C2O4-2

  • H+ + C2O4-2 ↔HC2O4-

  • Reduces C2O4-2 in acidic solution.


  • The reaction quotient (called ion product) may be applied to solubility equilibria - determines if a substance will precipitate from solution

  • Ion Product, Q =[M+]a[Nm-]b

  • If Ksp<Q a precipitate forms, reverse process occurs

  • If Ksp=Q equilibrium solution is just saturated

  • If Ksp>Q No precipitate, forward process occurs

Precipitation Example

  • A solution of 75.0 mL of 0.020 M BaCl2 is added to 125.0 mL of 0.040 M Na2SO4. Will a precipitate form? (Ksp= 1.5 x 10-9M BaSO4)

BaSO4 could form if Ksp<Q.

For Q you need initial concentrations:

[Ba2+] = mmol Ba2+ / total mL

= (0.0750L)(0.020 M)/(0.0750L + 0.125L) = 0.0075 M

[SO42-] = mmol SO42- / total mL

= (0.1250L)(0.040 M)/(0.0750L + 0.125L) = 0.025 M

Q = [Ba2+] [SO42-] = (0.0075 M)(0.025 M) = 1.9 x 10-4

Ksp<Qso BaSO4 will form.

To figure out concentrations set up an ice table.

Complex Ions

A Complex ion is a charged species composed of:

1. A metallic cation

2. Ligands – Lewis bases that have a lone electron pair that can form a covalent bond with an empty orbital belonging to the metallic cation

NH3, CN-, and H2O are Common Ligands

The Addition Of Each Ligand Has Its Own Equilibrium

  • Usually the ligand is in large excess.

  • And the individual K’s will be large so we can treat them as if they go to equilibrium.

  • The complex ion will be the biggest ion in solution.

Coordination Number

  • Coordination number refers to the number of ligands attached to the cation

  • 2, 4, and 6 are the most common coordination numbers

Complex Ions and Solubility

AgCl(s)  Ag+ + Cl- Ksp = 1.6 x 10-10

Ag+ + NH3 Ag(NH3)+ K1 = 1.2 x 10-3

Ag(NH3)+ NH3 Ag(NH3)2+ K2 = 9.6 x 10-4

K = KspK1K2

AgCl + 2NH3 Ag(NH3)2+ + Cl-

Dim the lights…

it’s showtime!


Pamukkale is one of the extraordinary natural wonders of Turkey.

The great attraction is the white immensity of the cliff with sculptured basins full of water and congealed waterfalls; they seem done of snow, cloud, cotton.

The scientific explanation is the hot thermal places that lie under the mount provoke the calcium carbonate spill, that makes the forms as solid as travertino marble.

One can bath there; the Turks call this place PAMUKKALE, which means "Castle of Cotton“.

It is a protecting landscape that fascinates, as the action of the mineral waters that contains calcium oxides left fantastic marks in the structures.

The resultant effect is spectacular: the waters spill on a series of steps, forming solid cascades and pools.

As much the cascades of calcium carbonate as the water change color in accordance with changes of the solar light that illuminates them, and the effect is surprising.

At times white, others blue, or green or other colors. The spectacle is flaring.

The continuous dynamics of the erosion and the transformation of the natural landscape result in an unusual environment.

PAMUKKALE is one of the most unique phenomena in nature.


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