Define (i) an acid, (ii) a base - Leaving Cert Chemistry - Question b - 2016

A base is a substance that gives hydroxide ions (OH⁻) in aqueous solution. It functions as a proton (H⁺ ion) acceptor.

The pH scale is used to quantify the acidity or alkalinity of a solution. It is calculated using the formula:

$pH = - ext{log}_{10}\left( \frac{[H^+]}{[H_2O^+]} \right)$

where [H⁺] is the concentration of hydrogen ions in mol/L.

For strong acids like nitric acid, it fully dissociates in solution. Thus, the concentration of H⁺ ions is equal to the concentration of the acid:

$[H^+] = 0.2 ext{ M}$

Then, using the pH formula:

$pH = - ext{log}(0.2) ≈ 0.70$

Nitric acid is a monoprotic acid, meaning it donates one proton per molecule. Sulfuric acid is diprotic and can donate two protons. Thus, for 0.2 M sulfuric acid:

1. It also dissociates completely in its first ionization, giving 0.2 M of H⁺.
2. For the second ionization, only a fraction contributes more H⁺ ions.

The effective concentration of H⁺ ions in 0.2 M sulfuric acid can be approximated to:

$[H^+] \approx 0.39 - 0.40 ext{ M}$

Therefore, since the pH of 0.2 M nitric acid is approximately 0.70 and that of sulfuric acid is calculated to be within the range of 0.39 - 0.40, nitric acid has the higher pH value.

Sulfuric acid is diprotic, meaning it can donate two protons, which leads to a higher overall concentration of hydrogen ions compared to nitric acid, which is monoprotic. Thus, while both are strong acids, the pH of sulfuric acid will be lower than that of nitric acid due to the higher availability of H⁺ ions from sulfuric acid.