I know the common strong and weak acids but if a mineral acid like $\ce{H3BO3}$ springs up, I can't actually tell whether it is strong or weak.
Also can I justify in chemical terms why $\ce{H2SO4}$ is a strong acid but $\ce{H2SO3}$ a weak one?  

Why is $\ce{HNO3}$ a strong acid whereas $\ce{HNO2}$ a weak one ?

<http://catalog.flatworldknowledge.com/bookhub/2273?e=ball-ch12_s04>

The above link shares a list of strong and weak acids but fails to state why one acid is weak while the other strong.