Write ionic equations that illustrate how each pair of compounds can serve as a buffer pair. Use the smallest possible integer coefficients.

H2CO3 and NaHCO3 adding H+,
AND
H2CO3 and NaHCO3 adding OH-,

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Write down how each of the two substances dissociate.

H_2CO_3 \rightleftharpoons CO_3\ ^{2-} + 2H^+

NaHCO_3 \rightarrow Na^+ + CO_3\ ^{2-} + H^+

Adding H^+ will react with the large 'reservoir' of CO3 2- from the sodium carbonate and form carbonic acid. Since the amount of carbonate from the acid is practically unchanged, the pH will remain practically the same.

Try with the other one now.