Why isn't
NO2 + H2O = H2NO3?
I've seen that the result should be HNO3 + NO [...]
You miss coefficients at the moles of reactants and products.
And: not every compound is possible. HNO
3, NO, NO
2, N
2O
4, N
2O
5... exist. I haven't heard about H
2NO
3.
This is hard to predict with N and O. You have to know them.