Which is the Lewis acid, and which is the Lewis base in the following equation: SO2(g) + H2O(l) --> H2SO3(aq)?
The Lewis acid is the electron pair acceptor; the Lewis base is the electron pair donor.
While it is not immediately obvious, it is clear that the sulfur species has accepted a lone pair from the water molecule; so sulfur dioxide is the Lewis acid in this example. A more old-fashioned treatment is to designate sulfur dioxide as the acid anhydride of sulfurous acid,
What do you think would be the result of the same reaction with sulfur trioxide,