Why is it that a substance such as CH3NH2 containing no OH- increases the pH when dissolved in water?

Question

and also, why is it that when SO3 is added to water the pH decreases?

Answer 1

As you know, a base will increase the pH of a solution. A base can be defined as a hydrogen ion acceptor or OH donor (Bronsted-Lowry definition), or as an electron pair donor (Lewis definition). CH3NH2 is an electron pair donor, and the NH2 group can accept a hydrogen ion to become NH3+. This increases the alkalinity of the solution.

Answer 2

This can be answered with a couple of equations:

CH3NH2 + H2O <------> (CH3NH3)+ + OH- (increase in pH)

SO3 + H2O -------> H2SO4 -----> 2H+ + (SO4)2- (decrease in pH)