A buffer consists of 0.16 M KHCO3 and 0.39 M K2CO3. Carbonic acid is a diprotic acid with Ka1 = 4.5 10-7 and Ka2 = 4.7 10-11.

Question

(a) Which Ka value is more important to this buffer?
Ka1
Ka2
I don't understand this one at all. How do I determine which Ka value is more important to the buffer?

Answer 1

A very good question that vexed me for years. Here is how I handle it.
H2CO3 ==> H^+ + HCO3^-
HCO3^- ==> H^+ + CO3^2-

k1 = (H^+)(HCO3^-)/(H2CO3)
k2 = (H^+)(CO3^2-)(HCO3^-)

and Henderson-Hasselbalch equation you will be using is
pH = pKa(1 or 2) + log(base)/(acid)
Look at the problem. The acid in the problem is HCO3^- and the base is CO3^2-
Does k1 have HCO3^- and CO3^2- in it? No. Does k2? Yes, it has CO3^2- in the numerator and HCO3^- in the denominator. Therefore, you use k2. That always makes it easy to spot. It is even more valuable when dealing with phosphate buffers because you have k1, k2, and k3 and you must choose which to use. This system always works.

Answer 2

Thank you so much!

Answer 3

Thank you for a very good explanation! :)

Answer 4

Thanks alot

Answer 5

But is it considered as acidic buffer or basic buffer solution?