Lewis acids and bases are defined as species that accept or donate electron pairs respectively. OH- is a Lewis base because it donates an electron pair to Al, which has empty orbitals and is thus a Lewis acid. SO2 and CaO reacting in water also demonstrate the Lewis acid-base concept, with SO2 forming the Lewis acid H2SO3 and CaO the Lewis base Ca(OH)2.

1. there are several theory on defining acid and base, Lewis acids and bases is the broadest. The
species donate electron pair is lewis base and the species accept electron pair is lewis acid. so for
the second one. the oxygen in OH- has 3 pair of electrons. It donate one pair to the Al which has
empty p-orbits thus Al accept one pair of electons. So OH- is lewis base, Al(OH)3 is lewis acid.
For the first one, it would be easier when you think their product in water solution. SO2 will give
H2SO3 and CaO will give Ca(OH)2, H2SO3 is clearly a brownsted acid(also lewis acid) and
Ca(OH)2 is a brownsted base(also lewis base).
Solution
there are several theory on defining acid and base, Lewis acids and bases is the broadest. The
species donate electron pair is lewis base and the species accept electron pair is lewis acid. so for
the second one. the oxygen in OH- has 3 pair of electrons. It donate one pair to the Al which has
empty p-orbits thus Al accept one pair of electons. So OH- is lewis base, Al(OH)3 is lewis acid.
For the first one, it would be easier when you think their product in water solution. SO2 will give
H2SO3 and CaO will give Ca(OH)2, H2SO3 is clearly a brownsted acid(also lewis acid) and
Ca(OH)2 is a brownsted base(also lewis base).