Answer:
HCO₂H
Explanation:
A buffer is defined as the mixture between a weak acid and its conjugate base, or vice versa, in an aqueous solution.
The HCO₂Na (Sodium formate), is the conjugate base of formic acid, HCO₂H, a weak acid of pKa = 3.75.
Thus, to make a buffer using HCO₂Na, the other substance the buffer should contain is HCO₂H
