I need help on finding the pKa value, I'm not sure how to use the hasselbatch equation to find it.

4. The pH of normal rainwater is approximately 5.6. The slight acidity compared to pure water (pH 7.0) is caused by dissolved CO2, as per the following reaction: CO2(av)+H2OH2CO3(as) In some regions of the world, the acidity of rain has increased to a pH of approximately 3.5. This is caused by the presence of polluted air containing SO2SO3, and NO2. These oxides react with rainwater to form acids, further lowering the pH. SO2+H2OH2SO3SO3+H2OH2SO42NO2+H2OHNO3+HNO2 With the lower pH, there is a higher proportion of bicarbonate present. Calculate the ratio of bicarbonate to carbonic acid in A) normal rain water (pH5.6) and in B) acid rain (pH3.5)(4pts)