To answer this problem, let us first define what a buffer solution is.
A buffer solution by definition is an aqueous solution consisting of a mixture of a weak acid and its conjugate base, or a weak base and its conjugate acid.
In this case, we have are going to have a mixture of a weak acid and its conjugate base. Sodium Formate or HCOONa is the conjugate base of that weak acid. Sodium formate is properly written as:
HCOO-Na+
We can find the weak acid by replacing Na+ with H+. Therefore, the other substance should be:
HCOO-H+
or
HCOOH
Answer:
Formic Acid = HCOOH