Phosphoric Acid vs Phosphorous Acid vs Hypophosphorous Acid: Understanding Acid Strength

Understanding Acid Strength

Phosphoric acid (H3PO4) is a triprotic acid with three acidic protons that can be ionized or donated, while phosphorous acid (H3PO2) is a diprotic acid with two acidic protons, and hypophosphorous acid (H3PO) is a monoprotic acid with only one acidic proton.

The difference in the number of acidic protons is due to the structure of the acid molecules. Phosphoric acid has three -OH groups bonded to a phosphorus atom, while phosphorous acid has two -OH groups, and hypophosphorous acid has only one -OH group.

When these acids ionize, they donate their acidic protons to form corresponding anions called phosphates. The ionization constants for each successive step of ionization decrease, resulting in weaker acid strength.

This phenomenon is a general characteristic of polyprotic acids, where the number of ionizable hydrogen atoms determines the acid's strength. The more acidic protons an acid can donate, the stronger the acid.