What is the pH of the solution that is 0.10 M H2CO3 and 0.20 M NaHCO3?
We'll assume the second acid dissociation of carbonic acid is negligible.
Since we have a weak acid and its conjugate base from a salt we have a buffer soln. For buffers solns use the Henderson-Hasselbalch eqn:
pH = pka + log (A- / HA)
where HA is any weak acid, and A- is its conjugate base.
For carbonic acid, ka = 4.3E-7
Plug and chug...
pH = -log (4.3E-7) + log (0.2/0.10)
= 6.67 (with sigfigs in mind)
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