Which of the following is a weak acid?

H3PO4, or phosphoric acid, is indeed classified as a weak acid. This classification is due to its incomplete ionization in water. When dissolved in water, it does not release all of its hydrogen ions (H+) fully, which is a characteristic feature of weak acids. Instead, it dissociates in a stepwise manner, meaning it can lose one hydrogen ion at a time, leading to a relatively lower concentration of ions compared to strong acids.

In contrast, H2SO4 (sulfuric acid), HCl (hydrochloric acid), and HNO3 (nitric acid) are strong acids. These acids completely ionize in solution, meaning they dissociate fully into their constituent ions when dissolved in water, resulting in a high concentration of hydrogen ions. The strength of an acid is often determined by its ability to donate protons, and strong acids effectively donate protons to the extent that they result in a near-complete ionization.

This understanding of how different acids behave in water is fundamental in the study of acid-base chemistry and lays the foundation for predicting reaction outcomes in various chemical environments.