No2 is Because HNO3 dissociates completely when dissolved in water, like this: HNO3 + H2O --> H3O+ + NO3-. H3O+ is called the hydronium ion. The more hydronium a substance produces when dissolved in water, the stronger the acid. This is what is called Arrhenius definition.
HNO2 dissociates partially, so it is what is called an equilibrium. The reaction is going back and forth between reactants and products, so we chemists represent the reaction in this form:
HNO2 + H2O <----> H3O+ + NO2-.
Since this reaction is going back and forth (think in terms of a see-saw in a playground), the H3O+ concentration never gets past certain point, it stays constant.
Why HNO3 is a strong acid and HNO2 is a weak one. To answer this question we have to look at the structures of the anions (negative charge species, in this case NO3- and NO2-). NO3- can be drawn by placing the N in the middle and drawing two single bonds to each oxygen and one double bond to the other, in a triangle. Which N-O bonds are single and which one in N=O double, is kind of arbitrary, so what that means is that the ion has resonance, the single bonds and the double bond are alternating sites, while for NO2- you can draw the N in the middle and one N-O and one N=O, so you have less choices. The ion with the more resonance structures (choices on how they are drawn), is more stable, so HNO3 actually prefers to loss the H+ and give it to water, so NO3- is formed, which is more stable than NO2-. I am an University Chemistry professor, so I know what I am talking about. Hope this helps.this is all that l know