My teacher said that the hydrogen which is directly linked with oxygen or more electronegative atom is an acidic hydrogen, and that the $n$-factor for an acid is the number of acidic hydrogens.
Then why is the $n$-factor of phosphinic acid $\ce{H3PO2}$ is 1? I saw its structure, all $\ce{H}$ atoms are linked to a more electronegative atom:
The structure of hypophosphorous acid is $\ce{HO-PH2=O}$. The electronegativity of phosphorus is near the same as for hydrogen. The oxygen causes slight polarization of $\ce{P-H}$ bond, making $\ce{H}$ slighly acidic, but acidity of the $\ce{H-P}$ is negligible in water environment.
Note that if putting rare exceptions and circumstances aside, we do not consider hydrocarbons as acids either. Even $\ce{NH3}$ is very weakly acidic, reacting very slowly with solvated electrons in liquid ammonia.
Note also that electronegativity is a synthetic parameter. Relative element electronegativities are not the decisive and only factor, but just a rough indicator of possible acidity. Compare e.g. acidity of hypochlorous acid $\ce{H-O-Cl}$ and perchloric acid $\ce{H-O-ClO3}$. It is clear that the effective shift of electron density (leading to bond polarization of acids) and charge delocalization of anionic conjugate bases have much bigger effect than the general element electronegativity.
| Element | Pauling electronegativity |
|---|---|
| $\ce{H}$ | 2.20 |
| $\ce{P}$ | 2.19 |
| $\ce{C}$ | 2.55 |
| $\ce{N}$ | 3.04 |
| $\ce{O}$ | 3.44 |
As Poutnik explains, phosphorus is not more electronegative than hydrogen, but "electronegativity" is not the real criterion for rendering an attached hydrogen atom acidic.
Electronegativity is essentially a combination of electron affinity with ionization energy, whereas making an attached hydrogen acidic is instead a combination of electron affinity minus bond strength between the element and hydrogen (if the bond with hydrogen is weak, the hydrogen may be dissociated as a proton even if the attached atom has low electron affinity, as is the case with iodine).
The net result is that the acidity of an attached hydrogen atom correlates with the group the atom attached to the hydrogen is in: Group 16 and 17 elements, whose electron configurations are just short of a noble gas, have relatively high electron affinities compared with their bond strengths to hydrogen, and it is these elements that then tend to acidic hydrogen atoms in water solvent.
So hydrogen attached to sulfur, let us say, is likely to be acidic in water solution, whereas hydrogen attached to nitrogen is not given the different group/electron configuration associated with nitrogen.
