Just as pH is the negative logarithm of [H3O+], pKa is the negative logarithm of Ka, pKA = - log KA The Henderson-Hasselbalch equation is used to calculate the pH of buffer solutions: PH = PKa+log[base]/[acid] Notice that the pH of a buffer has a value close to the pjRTa of the acid, differing only by the logarithm of the concentration ratio [base]/[acid]. The Henderson-Hasselbalch equation in terms of pOH and pKb is similar. pOH = pKb + log [acid]/[base] How many grams of dry NH4CI need to be added to 2.00L of a 0.300M solution of ammonia, NH3, to prepare a buffer solution that has a pH of 8.93? Kb for ammonia is 1.8 times 10-5.