Which of the following acids (listed with pKa values) and their conjugate base would form a buffer with a pH of 8.10?

(A) HC7H5O2, pKa = 4.19

(B) HF, pKa = 3.46

(C) HClO, pKa = 7.54

(D) HCN, pKa = 9.31

(E) HClO2, pKa = 1.96

(C) HClO, pKa = 7.54

Explanation:

A buffer is a solution that can resist abrupt changes in pH when acids or bases are added. It is formed by two components:

A weak acid and its conjugate base. A weak base and its conjugate acid.

In this case, acid and base are defined according to Bronsted-Löwry theory, which states that acids are substances that release H⁺ and bases are substances that accept H⁺. Therefore, when an acid loses an H⁺ transforms into its conjugated base. For example, HF/F⁻ is a conjugate acid-base pair.

In buffers, when an acid is added, it reacts with the base to diminish its amount:

F⁻ + H⁺ ⇄ HF

Also in buffers, when a base is added, it reacts with the acid to diminish its amount:

HF + OH⁻ = F⁻ + H₂O

The optimum pH range of work of a buffer system (known as buffer range) is between 1 unit less and 1 unit more of pH than its pKa.

So, the buffer formed by HClO/ClO⁻ works optimally in the pH range 6.54-8.54. Since pH = 8.10 is in that interval, this would be the optimal choice.