Several acids are listed here with their respective equilibrium constants:

C₆H₅OH(aq) + H₂O(ℓ) ⇄ H₃O⁺(aq) + C₆H₅O⁻(aq) K_a = 1.3 × 10⁻¹⁰

HCO₂H(aq) + H₂O(ℓ) ⇄ H₃O⁺(aq) + HCO₂⁻(aq) K_a = 1.8 × 10⁻⁴

HC₂O₄⁻(aq) H₂O(ℓ) ⇄ H₃O⁺(aq) + C2O₄²⁻(aq) K_a = 6.4 × 10⁻⁵

1. (a) Which is the strongest acid? Which is the weakest acid?
2. (b) Which acid has the weakest conjugate base?
3. (c) Which acid has the strongest conjugate base?

Interpretation: The strongest acid and the weakest acid has to be identified.

Concept introduction:

Strong acids dissociate completely in water which produce H+ ion, while weak acids dissociate partially in water and produce H+ ion.

An equilibrium constant (K) is the ratio of concentration of products and reactants raised to appropriate stoichiometric coefficient at equlibrium.

For the general acid HA,

  HA(aq)+H2O(l)⇌H3O+(aq)+A−(aq)

The relative strength of an acid and base in water can be also expressed quantitatively with an equilibrium constant as follows:

Ka=[H3O+][A−][HA]

An equilibrium constant (K) with subscript a indicate that it is an equilibrium constant of an acid in water.

For acetic acid (HCOOH),

  HCOOH(aq)+H2O(l)⇌H3O+(aq)+HCOO−(aq)

Ka=[H3O+][HCOO−][HCOOH]

A large value of Ka=1.8×10−4 indicates that the ionization product is strongly favoured because the product [H3O+][HCOO−] is more than the equilibrium concentration of the strong acid [HCOOH].

For phenol (C6H5OH),

    C6H5OH(aq)+H2O(l)

Interpretation: The weakest conjucate base has to be identified.

Concept introduction:

Bronsted –Lowry conjugated acid-Base pairs:

When an acid is dissolved in water, the acid (HA) donates a proton to water to form a new acid (conjugated acid) and a new base (conjugated base).

The pair of an Acid –Base differs by a proton called conjugated Acid-Base pair.

Interpretation: The strongest conjucate base has to be identified.

Concept introduction:

Bronsted –Lowry conjugated acid-Base pairs:

When an acid is dissolved in water, the acid (HA) donates a proton to water to form a new acid (conjugated acid) and a new base (conjugated base).

The pair of an Acid –Base differs by a proton called conjugated Acid-Base pair.

An equilibrium constant (K) is the ratio of concentration of products and reactants raised to appropriate stoichiometric coefficient at equlibrium.

For the general acid HA,

  HA(aq)+H2O(l)⇌H3O+(aq)+A−(aq)

The relative strength of an acid and base in water can be also expressed quantitatively with an equilibrium constant as follows:

Ka=[H3O+][A−][HA]

An equilibrium constant (K) with subscript a indicate that it is an equilibrium constant of an acid in water.