Which one is a weak acid?

Concept: Acids are classified as strong or weak based on how completely they ionise in water. A strong acid dissociates almost completely into H+ and its conjugate base in dilute aqueous solution, whereas a weak acid dissociates only partially, setting up an equilibrium between the undissociated molecule and its ions (a small dissociation constant, Ka).

Application: Citric acid is a weak, polyprotic organic acid naturally present in citrus fruits. In water it ionises only partially, so a solution of citric acid always contains a large fraction of undissociated citric acid molecules alongside H+ ions and citrate ions.

By contrast, the other three acids offered are classic strong (mineral) acids that ionise almost completely in dilute aqueous solution:

• Nitric acid (HNO3) ionises almost completely in dilute aqueous solution, making it a strong acid.

• Sulphuric acid (H2SO4) also ionises almost completely in its first step in dilute aqueous solution, making it a strong acid.

• Hydrochloric acid (HCl) ionises essentially completely in water, making it a strong acid.

Cross-check: This matches the dissociation constants: citric acid has small Ka values (of the order 10-3 to 10-6 for its three ionisable protons), while nitric, sulphuric, and hydrochloric acid all have a degree of ionisation approaching unity in dilute solution, confirming citric acid as the one weak acid among the four options.