# What is "orthophosphoric acid"?

..........and you might as well ask why the molecular formula of methane is $C {H}_{4}$, or the molecular formula of sodium chloride is $N a C l$.
${H}_{3} P {O}_{4}$, $\text{phosphoric acid}$, in the old literature $\text{orthophosphoric acid}$, is a fairly weak mineral acid, known to form equilibrium quantities of ${H}_{2} P {O}_{4}^{-}$ and $H P {O}_{4}^{2 -}$ in aqueous solution. The third dissociation to give $P {O}_{4}^{3 -}$ can usually be ignored in aqueous solution.