Answer:
D. [HCHO₂] > [NaCHO₂]
Explanation:
Formic acid, HCHO₂, is a weak acid that, in presence of its conjugate base, NaCHO₂ (CHO₂⁻), produce a buffer following H-H equation:
pH = pKa + log [CHO₂⁻] / [HCHO₂]
As pKa of the acid is 3.74 and pH of the solution is 3.11:
3.11 = 3.74 + log [CHO₂⁻] / [HCHO₂]
-0.63 = log [CHO₂⁻] / [HCHO₂]
0.2344 = [CHO₂⁻] / [HCHO₂]
A ratio [CHO₂⁻] / [HCHO₂] < 1, means:
[HCHO₂] > [CHO₂⁻]
