9 For a n acid HA the concentrations of HA and A are 01M and

9. For a n acid, HA, the concentrations of HA and A are 0.1M and 0.02 M respectively at pH 4.0. What is the pKa value for HA?

Given pH =4.0 & conc of A^- =0.02 M , conc of HA = 0.1 M

So according to Henderson hasselbalch equation

pH=pKa + log [A^- / HA]

4.0 = pKa + log [ 0.02 / 0.1 ]

4.0 = pKa + log [ 0.2 ]

4.0 = pKa -0.699

pKa = 4.0 + 0.699

pKa = 4.699