You're right to point out the difference in strength between phosphoric acid (H₃PO₄) and hydrochloric acid (HCl), even though they have different numbers of hydrogen atoms. Here's why:

The strength of an acid is determined by its ability to donate a proton (H+) in solution.

* HCl is a strong acid because it readily donates its single proton to water, forming H₃O+ (hydronium ion) and Cl- (chloride ion). This dissociation is essentially complete in solution, meaning most HCl molecules donate their proton.

* H₃PO₄ is a weak acid because it only partially donates its protons in solution. It undergoes a series of ionization steps, with each subsequent proton being more difficult to remove. The first ionization step produces H₂PO₄- (dihydrogen phosphate ion), the second step produces HPO₄²⁻ (hydrogen phosphate ion), and the final step produces PO₄³⁻ (phosphate ion). Each step has its own equilibrium constant (Ka), and the Ka values decrease with each ionization, indicating that the removal of each subsequent proton is less favorable.

Here's the key:

* Strength of an acid is not directly related to the number of hydrogen atoms.

* It depends on the stability of the conjugate base formed after the proton is donated.

In the case of HCl, the Cl- ion is very stable, making the dissociation of HCl very favorable.

In the case of H₃PO₄, the conjugate bases (H₂PO₄⁻, HPO₄²⁻, and PO₄³⁻) are less stable due to the negative charge distribution over multiple oxygen atoms. This makes it more difficult for the acid to lose its protons and explains why H₃PO₄ is a weak acid.

Let me know if you would like more detail on the factors that influence acid strength!