Question

A buffer of pH 9.24 is to be prepared from a weak acid and its salt. The best acid from which to prepare the buffer is

(A) phthalic acid, K1 = 1.3 x 10-3 (1st ionization)
(B) hydrohen phthalate, K2 = 3.9 x 10-5
(C) arsenious acid, Ka = 6 x 10-10
(D) formic acid, Ka = 1.8 x 10-5
(E) phenol, Ka = 1.3 x 10-10

Answer 1

(C) arsenious acid, Ka = 6 x 10⁻¹⁰

Step-by-step 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.