The most effective buffer solutions have equal concentrations of salt and acid (or base). Find the pH of a buffer solution with equimolar concentrations of:

Question

(a) HCOOH and NaCOOH (Ka = 3.5 x 10-4 for HCOOH)


(b)CH3NH2 and CH3NH3Cl (Kb = 3.7 x 10-4 for CH3NH2)

Thank you!

Answer 1

When equimolar concentrations of base and acid are used, the pH is ALWAYS the pKa. Why?
pH = pKa + log (base)/(acid)
But if base = acid, then base/acid = 1 and log 1 = 0 so pH = pKa.
a is pKa = - log Ka
b is pKb = -log Kb,
then pKa + pKb = 14