What substance would you add to nahco3(aq) to form a buffer solution?

Buffer solution is made by mixing a weak acid with its conjugate base or weak base with its conjugate acid. NaHCO₃(aq) is a conjugate salt and also a conjugate base of H₂CO₃(aq) acid. Hence H₂CO₃(aq) should be added to make the buffer solution.

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