Why do dihydrogen phosphate salts make weakly acidic solutions?

Dihydrogen phosphate salts make weakly acidic solutions because they contain the dihydrogen phosphate ion (${\text{H}}_2{\text{PO}}_4^{-}$), which acts as a weak acid in aqueous solution.1

Orthophosphoric acid is a triprotic acid, meaning it can donate three protons in succession. When these salts dissolve in water, the ${\text{H}}_2{\text{PO}}_4^{-}$ ion is capable of donating a proton, resulting in an acidic environment. In contrast, other phosphate salts behave differently based on their specific ion composition: hydrogen phosphate ions (${\text{HPO}}_4^{2-}$) act as bases because they are stronger proton acceptors than proton donors, while orthophosphate ions (${\text{PO}}_4^{3-}$) are moderately strong bases that produce strongly basic solutions.1