In a titration of 100 mL 0.10M nitrous acid HNO2 against 0.10M sodium hydroxide, calculate the pH at the following stages:
K(a) for the nitrous acid = 4.5 x 10-4
a) before any base has been added to reach the acid solution.
b) after 25% of the base necessary to reach the equivalence point has been added.
c) at the equivalence point.