pKa1 and pKa2 of H2SO3 are 1,.82 and 7.20. pH after 20 mL of 0.160 M NaOH is added to 40 mL of 0.080 IM H2SO3 is

H2SO3+NaOH⇌NaHSO3+H2O 20mL of 0.100MNaOH≡3.2 millimoles of NaOH  Thus, H2SO3 is converted into NaHSO3,HSO3−is amphiprotic  H+ion donor as well as acceptor)  HSO3−⇌H++SO32− HSO3− Base +H+⇌H2SO4 pH=pKa1+pKa22=1.82+7.202=4.51