Which one of the following acids has the highest pH value ?

The pH value is a measure of the hydrogen ion concentration [H⁺] in a solution, defined as pH = -log[H⁺]. A higher pH value indicates a lower concentration of hydrogen ions, characteristic of a weaker acid.

• Nitric acid (HNO₃), Sulphuric acid (H₂SO₄), and Hydrochloric acid (HCl) are strong acids. They dissociate almost completely in water, producing high concentrations of H⁺ ions and resulting in very low pH values (near 0 or even negative for 1 M concentrations).
• Acetic acid (CH₃COOH) is a weak organic acid. It only partially ionizes in aqueous solution. At 1 M concentration, its [H⁺] is much lower (approx. 0.0042 M) compared to strong acids, resulting in a significantly higher pH value (approximately 2.4).

Since Acetic acid has the lowest concentration of hydrogen ions among the options, it has the highest pH value.