Define an acid in terms of the Brønsted-Lowry theory - Leaving Cert Chemistry - Question b - 2012

A conjugate pair consists of an acid and its corresponding base that differ by a proton. When an acid donates a proton, it becomes its conjugate base, and vice versa.

The colour observed when a few drops of the indicator solution are added to a 0.5 M NaOH solution is purple. This occurs because hydroxide ions (OH⁻) from the NaOH remove H⁺ ions from HIn, causing the equilibrium of the dissociation reaction to shift to the right, where In⁻ predominates, resulting in the purple colour.

To calculate the pH of the 0.5 M NaOH solution, we first calculate the pOH:

$pOH = - ext{log}[OH^-] = - ext{log}(0.5) \ = 0.301$

Then, we find the pH using:

$pH = 14 - pOH = 14 - 0.301 = 13.699$

To calculate the pH of a 0.1 M solution of the indicator:

First, we find the concentration of H⁺ ions using the formula for Ka:

$Ka = [H^+][In^-] / [HIn] = 2.0 × 10^{-3}$

At equilibrium, let us assume [H⁺] = [In⁻] and hence:

Ka = rac{[H^+]^2}{0.1 - [H^+]}

With [H⁺] much smaller than 0.1: