What is the pH of a buffer?

Question

The K_a of H₂CO₃ is 4.3 x 10^-7. What is the pH of a buffer with 0.050 M H₂CO₃ and 0.50 M NaHCO₃?

Answer 1

We can use the Henderson Hasselbalch equation:

pH = pKa + log [salt]/[acid]

pKa = -log Ka = -log 4.3x10-7 = 6.37

pH = 6.37 + log (0.5/0.05) = 6.37 + log 10

pH = 6.37 + 1

pH = 7.37