Question

If the pKaof HCHO2is 3.74 and the pH of an HCHO2/NaCHO2solution is 3.11, which of the following is true?

A. [HCHO2] < [NaCHO2]
B. [HCHO2] = [NaCHO2]
C. [HCHO2] << [NaCHO2]
D. [HCHO2] > [NaCHO2]
E. It is not possible to make a buffer of this pH from HCHO2 and NaCHO2

Answer 1

D. [HCHO₂] > [NaCHO₂]

Step-by-step 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⁻]