Solubility Equilibria

The equilibrium constant that represents the maximum amount of solid which can be dissolved in an aqueous solution is defined as the solubility product. The Solubility Equilibria are based on the assumption that the solids dissolve in water in order to give the basic particles from which they are formed.

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Solubility Equilibria

Each of the molecular solids dissolves to give an individual aqueous molecule such as

H₂O
C₁₂H₂₂O_11(S) —————> C₁₂H₂₂O_{11 (aq)}

The ionic solids dissociate to give their respective positive and negative ions :

H₂O
NaCl _(s) ————————-> Na + (aq) + Cl ^– (aq )

The ions formed from the dissociation of the ionic solids can also carry an electrical current which makes the salt solutions good conductors of electricity. However, molecular solids do not dissociate in water to give ions so as no electrical current can be carried.

Browse more Topics under Equilibrium

• cids, Bases and Salts
• Buffer Solutions
• Equilibrium in Chemical Processes
• Equilibrium in Physical Processes
• Factors Affecting Equilibria
• Ionization of Acids and Bases
• Law of Chemical Equilibrium and Equilibrium Constant

Solubility

The ratio of the maximum amount of solute to the volume of the solvent in which the solute can dissolve. This is generally expressed in two ways i.e –

1. Grams of solute per 100 g of water
2. Moles of solute per litre of solution

A salt refers to be soluble if it dissolves in water to give a solution along with the concentration of at least 0.1 m at the room temperature. A salt is also considered to be insoluble if the concentration of an aqueous solution is less than 0.0001 m at the room temperature. Salts are considered to be slightly soluble; those between 0.0001 m and 0.1 m.

According to the principles of solubility equilibria, the salts have low solubilities in water. The reaction that can be considered for the dissociation of the salt AgCl is –

AgCl_(s) ————— Ag⁺_(aq) + Cl^– _(aq)

However, the reverse reaction for the dissolving of the salt would be the precipitation of the ions to form a solid –

Ag⁺_(aq) + Cl^– _(aq) —————–  AgCl(s)

When the rate at which AgCl dissolves is equal to the rate at which AgCl precipitates, the system has reached the solubility equilibria.

Solubility Product Equilibrium Constant (K_sp)

The product of the equilibrium concentrations of the ions in a saturated solution of a salt.  Each concentration in this is raised to the power of the respective coefficient of ion in the balanced equation. For example, the solubility product equilibrium constant for the dissociation of AgCl is:

Another example of a solubility product equilibrium constant where we can consider the reaction for the dissociation of CaF₂ in water is :

The solubility product equilibrium constant for this reaction would be the product of the concentration of Ca²⁺ ion and the concentration of the F^– ion raised to the second power (squared):

Solved Examples for You

Question: In the atmosphere, SO₂ and NO are oxidised to SO₃ and NO₂, respectively, which react with water to give H₂SO₄ and HNO₃. The resultant solution is called acid rain. SO₂ dissolves in water to form diprotic acid.

SO_2(g)+H₂O_(l)⇔HSO^⊝₃+H^⊕; K_a1=10⁻²

HSO^⊝₃⇔SO₂⁻³+H^⊕; K_a2=10⁻⁷

and for equilibrium,

SO_2(aq)+H₂O_(l)⇔SO₂⁻³_(aq)+2H^⊕_(aq)

K_a=K_a1×K_a2=10^-9 at 300K.

1. H₂SO₃ is less acidic than H₂SO₄
2. HNO₃ is less acidic than HNO2
3. SO_2(g) is reduced in the atmosphere during a thunderstorm
4. CO₂ gas develop more acidity in water than SO₂

Solution: Option A. Sulphurous acid H₂SO₃ is less acidic than sulphuric acid H₂SO₄. When a proton is lost by sulphurous acid, the negative charge is delocalized on 3 O atoms and when a proton is lost by sulphuric acid, the negative charge is delocalized by 4 O atoms. Since the extent of delocalization in HSO^-1 ion is less than the extent of delocalization in HSO^-4, the stability of anion and the acidity of the acid in sulphurous acid is lower than in sulphuric acid.