Answer
$HNO_3$ is stronger than $HNO_2$, because it has more oxygens connected to the central atom $(N)$.
Work Step by Step
- This happens because, the greater number of oxygens increases the stability of the conjugate base, because the negative charge is going to be more distributed, making it more stable.
More stable conjugate base : stronger acid.
- The extra oxygens also increases the polarity of the $O-H$ bond, by pulling electrons, due to the $O$ electronegativity.
