9. (a) Define acid according to the theory of - Leaving Cert Chemistry - Question 9 - 2015

A Bronsted-Lowry acid is defined as a proton (H⁺) donor. This means that an acid is any substance that can donate a hydrogen ion to a base in a chemical reaction.

pH is a logarithmic scale used to specify the acidity or basicity of an aqueous solution. It is defined mathematically as:

$pH = - ext{log}[H^+]$

where $[H^+]$ is the concentration of hydrogen ions in moles per liter.

One limitation of the pH scale is that it is only reliable for dilute solutions. The scale can become inaccurate in highly concentrated solutions where the activity of ions differs significantly from their concentration.

The pH curve for the titration should be accurately plotted on graph paper, with volume of NaOH on the x-axis and pH on the y-axis. The curve will typically show a steep increase as the equivalent point is approached, indicating a rapid change in pH.

Using the expression for the dissociation of the acid, we can set up the relationship as:

$K_a = \frac{[H^+][CH_3COOH]}{[CH_3COO^-]}$
Taking the concentration of H⁺ corresponding to the pH value:

$\text{If } pH = 3.3, \text{ then } [H^+] = 10^{-3.3} ext{ mol/L}$

Now substituting into the expression, we can solve for the concentration of ethanolic acid:

Letting $[CH_3COOH]$ denote the concentration we find:

$[CH_3COOH] = 0.014 ext{ M}$

Phenolphthalein is a suitable indicator for this titration as it changes from colorless to pink at a pH around 8.2, which is relevant to the pH range at the endpoint of the titration between an acid and a strong base.