Phosphoric acid is stronger than phosphoric acid
The strength of acid is related to many factors, and phosphoric acid is stronger than phosphoric acid.

Phosphoric acid, in its structure, there are two hydroxyl groups connected to the phosphorus atom, and another hydrogen atom is directly connected to the phosphorus atom. For phosphoric acid, although there are three hydroxyl groups connected to the phosphorus atom, the acidity of the two is different.

First of all, the electronic effect. The hydrogen atom directly connected to phosphorus in phosphoric acid can produce the induction effect of pushing electrons because its electronegativity is lower than that of phosphorus. This push electron effect increases the density of the electron cloud of the phosphorus-oxygen bond, but at the hydroxyl group, the electron cloud density of the oxygen-hydrogen bond decreases. After the electron cloud density of oxygen-hydrogen bonds decreases, hydrogen atoms are more likely to dissociate in the form of protons, thus enhancing acidity. In contrast, phosphoric acid, although there are three hydroxyl groups, there is no special electronic effect generated by hydrogen atoms directly connected to phosphorus. The interaction between hydroxyl groups makes the electron cloud distribution relatively stable, and the dissociation of oxygen-hydrogen bonds is relatively difficult.

Furthermore, in terms of the stability of conjugated bases. The conjugated base formed after the loss of protons of phosphoric acid can undergo a certain degree of resonance due to the existence of phosphorus-hydrogen bonds, so that the negative charge can be dispersed. The higher the degree of negative charge dispersion of the conjugated base, the stronger its stability, the stronger the ability of the corresponding acid to give protons, and the stronger the acidity. The conjugated base formed after phosphoric acid loses protons, although the hydroxyl oxygen atoms can disperse part of the negative charge through resonance, in comparison, the degree of dispersion is not as good as that of the conjugated base of phosphoric acid. Because the phosphorus and hydrogen bonds in the conjugated base of phosphoric acid participate in the resonance, the negative charge dispersion range is wider, so the acidity of phosphoric acid is stronger than that of phosphoric acid.

From this perspective, phosphoric acid is more acidic than phosphoric acid due to its special structure, electronic effect and stability of conjugated bases.