Welcome to our

presentation on
Solubility and Complex
ion Equilibria
 Group 3

Solubility and Presented by-

Abdullah Al Mahmud -190516
Complex ion Abdullah Al Robin -190518
Puja Seath -190519
Equilibria Tanzir Wahid Hasib -190520
Sumaya Tabassum Oyshi -190522
Md Shakil Ahmed -190523
Mia Din Arif Hasan -190524
 Solubility and Contents
Complex ion Equilibria Solubility and common ion effect. 4-6

The solubility product constant 7-9

Precipitation Calculations 10-12

Effect of pH on Solubility 13-14

Complex ion formation 15-17

Complex ion & Solubility 18-20

Qualitative analysis of metal ions 21-24
Topic 1
Solubility and common ion effect

Solubility:
The highest amount of a substance that will dissolve in equilibrium in a specified
volume of solvent at a particular temperature. It refers to the ability of a
substance to dissolve.

Common ion:
A common ion is an ion that is in common to both salts in a solution.

In a saturated solution of silver chloride, the equilibrium state,

AgCl s ⇌ Ag + + Cl−
When sodium chloride (NaCl) is added to the solution, the Cl− ions will increase.

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Topic 1
Solubility and common ion effect
Common ion Effect
Degree of Dissociation:
The fraction of the amount of the electrolyte in solution present as free ions is
called the degree of dissociation.

amount dissociated (mol/L)

𝑥=
initial concentration (mol/L)

In case of, AB ⇌ A+ + B−
A + B−
Degree of dissociation, 𝑥=
AB

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Topic 1
Solubility and common ion effect
Common ion Effect
The reduction of the degree of dissociation of a salt by the addition of a common-
ion is called the Common-ion effect.
Example:
In a saturated solution of silver chloride, the equilibrium state,
AgCl s ⇌ Ag + + Cl−
When sodium chloride (NaCl) is added to the solution, the Cl− ions will increase.
So the equilibrium will be shifted to the left to form more AgCl.

Thus the solubility of AgCl will decrease.

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Topic 2
The solubility product constant
Solubility information can be provided as the solubility (S) or as the solubility
product constant(Ksp).

Solubility product constant:

The solubility product constant is the equilibrium constant for a solute dissolving in
a solution. It represents the level at which a solute dissolves in solution.

Uses of Solubility product constant (Ksp):

 Describe the saturated solution of ionic compound.
 Calculate the effect of a common ion or pH on solubility.

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Topic 2
The solubility product constant
Calculating the solubility product constant (Ksp):
Ksp is also the equilibrium constant. So we need the dissociation equation of a
reaction.
Let’s consider a equation,
AB ⇌ A+ + B−
Dissociation equation = [𝐴+ ] × [𝐵− ]
So the solubility product constant, Ksp = 𝐴+ × 𝐵−

For example,
AgCl ⇌ Ag + + Cl−
Ksp = Ag + × Cl−

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Topic 2
The solubility product constant
Example of Solubility Product Constant

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Topic 3 Precipitation Calculations

Precipitation:
 Precipitation is the process of conversion of a chemical substance into a solid
from a solution by converting the substance into an insoluble form or a super-
saturated solution.

 When the reaction occurs in a liquid solution, the solid formed is called the
precipitate.

 The chemical agent that causes the solid to form is called the precipitant.

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Topic 3 Precipitation Calculations
Mathematically, this can be described using the Q value.
Q (ion product quotient): The product of the equilibrium expression when the
system is not at equilibrium.
Calculating Q for the equation: 𝐵𝑎𝑆𝑂4 ⇌ 𝐵𝑎2+ + 𝑆𝑂4 2−
𝑄 = 𝐵𝑎2+ 𝑆𝑂4 2−
If Q > Ksp; a precipitate will form (until Q decreases to Ksp.)
If Q = Ksp; equilibrium is reached - no precipitate will form.
If Q < Ksp; any soluble in solution will dissolve until Q increases to Ksp.

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Topic 3 Precipitation Calculations
Mathematical Problem:
The concentration of lead ion is 0.25 M. If the concentration of chloride ion is 0.0060
M, do you expect lead chloride to precipitate? Ksp for lead chloride is 1.7 × 10−5
Solution:
𝑃𝑏𝐶𝑙2 𝑠 ⇌ 𝑃𝑏 2+ 𝑎𝑞 + 2𝐶𝑙 − 𝑎𝑞
0.25M 0.006 M
𝑄 = 𝑃𝑏 2+ 𝐶𝑙 − 2
𝑄 = 0.25 × 0.0062
𝑄 = 9.0 × 10−6
Q < Ksp; indicating that a no precipitate will form.

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Topic 4
Effect of pH on Solubility
 Not only will solubility be affected by a common ion, but it can also be affected
by pH.
 This occurs when one of the ions in the salt is an acid or base.

Example:
Effect of pH on Mg(OH)2 solubility:
• In water, K sp = 8.9 × 10−12
• Mg(OH)2 ⇌Mg 2+ + 2 (OH)−

When base is added, solubility will decrease.

When acid is added, solubility will increase.

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Topic 4
Effect of pH on Solubility
Effect of pH on Al(OH)3 solubility:
• In water, K sp = 1.9 × 10−33
• Al(OH)3 (s)⇌Al3+ 𝑎𝑞 + 3 𝑂𝐻 − 𝑎𝑞
When base is added, solubility will decrease.
When acid is added, solubility will increase.

Effect of pH on AgCl solubility:

• AgCl(s) ⇌ Ag + 𝑎𝑞 + Cl− 𝑎𝑞
Cl− is a weak base
pH will not affect on AgCl solubility.

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Topic 5
Complex ion formation
Complex ion:
A complex ion is an ion comprising one or more ligands attached to a central metal
cation with a dative bond.

There is a bit of unique nomenclature to complex ion.

 The metal ion is known as the central metal ion.

 The anions or molecules attached to the metal is called ligands.

Example of Ligands: 𝑁𝐻3 , 𝑂𝐻 − , 𝑁𝑂+

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Topic 5
Complex ion formation
There is a bit of unique nomenclature to complex ion (Continued):
 The co-ordination number is the number of places on the metal ion where ligands
are bound.
Example of co-ordination number: Ag + 𝑎𝑞 + Cl− 𝑎𝑞 ⇌ AgCl(s)
Co-ordination number: 2

 The bond between the metal and ligand is known as co-ordinate covalent bond.

Examples: [Fe(H2O)6]2+, [CoCl4]2-, [Cu(NH3)4(H2O)2]2+

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Topic 5
Complex ion formation
Formation of complex ion:
Formation of a chemical complex has an effect on solubility.

A well-known example is the addition of a concentrated solution of ammonia

(𝑁𝐻3 ) to a suspension of silver chloride (AgCl).

Ag + aq + NH3 aq ⇌ Ag (NH3 )+ aq
Ag (NH3 )+ aq + NH3 aq ⇌ Ag (NH3 )2 + aq

The overall equation for the formation,

Ag + aq + 2NH3 aq ⇌ Ag (NH3 )2 + aq

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Topic 6
Complex ion & Solubility

Complex ion & Solubility:

Ionic solids that are nearly water-insoluble must be dissolved somehow in aqueous
solutions.

Example - When the various qualitative analysis groups are precipitated out, the
precipitates must be redissolved to separate the ions within each group.

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Topic 6
Complex ion & Solubility

Strategies for Dissolving a Water-Insoluble Ionic Solid:

 If the anion of the solid is a good base, the solubility is greatly increased by
acidifying the solution

 When the anion is not sufficiently basic, the ionic solid can be dissolved in a
solution containing a ligand that forms stable complex ions with its cation

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Topic 6
Complex ion & Solubility

Strategies for Dissolving a Water-Insoluble Ionic Solid: (Continued…)

 Some insoluble solids have to go through a combination of reactions to be
dissolved

 The solubility of many salts increases with temperature

• Simple heating is enough to make a salt sufficiently soluble

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Topic 7
Qualitative analysis of metal ions

Selective Precipitation:
 Step 1 : Add dil. HCl to the solution of the mixture. Ag + will precipitate as
AgCl. It is removed by filtration.

 Step 2 : Pass H2 S through the filtrate. Cu2+ is precipitated as CuS and

removed by filtration.

 Step 3 : Add NH4OH or NaOH solution to filtrate. Fe3+ is precipitated as

Fe(OH)3

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 Topic 7
Qualitative analysis of metal ions

Precipitation into the

qualitative analysis groups.

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Topic 7
Qualitative analysis of metal ions
Separation of the Basic ions into Groups

 Group I — Insoluble Chlorides.

 Group II — Sulphides insoluble in acid solution

H2 S ⇌ 2H+ + S2− (equilibrium shifted to left)

 Group III — Insoluble Hydroxide in NH4 OH + NH4 Cl

NH4 OH ⇌ NH4 + + OH−

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Topic 7
Qualitative analysis of metal ions
Separation of the Basic ions into Groups (Continued…):
Group IV — Insoluble Sulphides in basic solution
H2 S ⇌ 2H + + S2−

Group V — Insoluble Carbonates

BaCO3 ⇌ Ba2+ + CO3 2−
(excess)

Group VI. The cations 𝑁𝑎+ and 𝐾 + , if present in the mixture, are identified by
individually testing by the flame test.

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 The End

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