Define an acid according to the theory of (i) Arrhenius, (ii) Brønsted and Lowry - Leaving Cert Chemistry - Question 7 - 2021

One limitation of Arrhenius' theory is that it does not account for acid-base reactions that occur in non-aqueous solvents, or the behavior of acids that do not produce H⁺ in solution.

A conjugate acid-base pair consists of two substances that differ by the presence of a proton (H⁺). For example, in the pair (HA, A⁻), HA is the acid that can donate a proton to become A⁻, the conjugate base.

A strong acid completely dissociates in solution, producing a high concentration of H⁺ ions, while a weak acid only partially dissociates, resulting in a lower concentration of H⁺ ions and an equilibrium between the acid and its ions.

The balanced equation is:

$HSO_4^{-} + H_2O \rightleftharpoons H_3O^{+} + SO_4^{2-}$

$HA + H_2O \rightleftharpoons H_3O^{+} + A^{-}$

Given that HA is 1.5% dissociated in a 0.10 M solution:

Concentration of H₃O⁺: $[H_3O^{+}] = [A^{-}] = 0.10 M \times 0.015 = 0.0015 M$

Using the formula: $pH = -\log[H_3O^{+}] = -\log(0.0015) \approx 2.82$

Using the formula for Ka: