Buffer Solution Chemistry Math Question 8

Question

100 mL of 0.05 M carbonic acid is titrated with strong base to its second equivalence point, which occurs when 240.0 mL of the base has been added. What is the pH of solution at the second equivalence point? The acid dissociation constants for carbonic acid are pK₁=6.35, pK₂=10.33

Answer 1

Using the simplest approach for an amphiprotic salt, we can find the pH as follows:

pH = 1/2 (pKa1 + pKa2)

pH = 1/2 (6.35 + 10.33)

pH = 1/2 (16.68)

pH = 8.34

(NOTE: it matters not what the concentration of H2CO3 is, as long as it is not extremely low, in which case the autoionization of water would have to be included, and the calculation would be more tedious)