Phosphoric acid is a weak acid because it only partially dissociates in water. When phosphoric acid dissolves in water, it reacts with water molecules to form hydrogen ions (H+) and dihydrogen phosphate ions (H2PO4-). However, some of the phosphoric acid molecules remain undissociated in the solution.

The extent to which phosphoric acid dissociates is determined by its dissociation constant, which is a measure of the strength of the acid. The dissociation constant for phosphoric acid is 7.5 x 10-3, which means that only a small fraction of the phosphoric acid molecules dissociate in water.

The weakness of phosphoric acid is also due to the fact that the dihydrogen phosphate ions that are formed are relatively strong bases. This means that they can react with hydrogen ions to form undissociated phosphoric acid molecules. The combination of these factors makes phosphoric acid a weak acid.