Answer:
(C) arsenious acid, Ka = 6 x 10⁻¹⁰
Explanation:
A buffer is prepared by a weak acid and the conjugate base coming from its salt. Its function is to resist abrupt changes in pH when an acid or a base are added. The best working range of a buffer is in the range of pKa ± 1. Let's consider the 5 options and their pKa (pKa = -log Ka).
(A) phthalic acid, K1 = 1.3 x 10⁻³ (1st ionization) pKa = 2.9
(B) hydrogen phthalate, K2 = 3.9 x 10⁻⁵ pKa = 4.4
(C) arsenious acid, Ka = 6 x 10⁻¹⁰ pKa = 9
(D) formic acid, Ka = 1.8 x 10⁻⁵ pKa = 4.7
(E) phenol, Ka = 1.3 x 10⁻¹⁰ pKa = 9.8
The acid whose pKa is closer to the desired pH is arsenious acid. Its working range of pH is 8 - 10. In the second place, phenol could work as a buffer system since the working pH range is 8.8 - 10.8.
