Which of the following conjugate acid-base pairs should you...

Which of the following conjugate acid-base pairs should you choose to prepare a buffer solution that has
${pH}=4.50 ?$ Explain.
(a)${HSO}_{4}^{-}$ and ${SO}_{4}^{2-}$ (b) ${HOCl}$ and ${OCl}^{-}$
(c) ${C}_{6} {H}_{5} {CO}_{2} {H}$ and ${C}_{6} {H}_{5} {CO}_{2}$

Key Concepts

Acid Dissociation Constant (pKa)

The acid dissociation constant, represented as pKa, is a measure of the strength of a weak acid in solution, indicating the pH at which half of the acid is dissociated into its conjugate base. For buffer preparation, it is desirable that the pKa of the acid is near the target pH, ensuring that the buffer has significant amounts of both the acid and its conjugate base to effectively absorb added acids or bases.

Buffer Solutions

A buffer solution is a system that minimizes changes in its pH when small amounts of acid or base are added. Buffers are typically composed of a weak acid and its conjugate base or a weak base and its conjugate acid, working together to neutralize added H+ or OH- ions. Their ability to resist pH changes makes them essential in many chemical, biological, and industrial processes.

Conjugate Acid-Base Pairs

Conjugate acid-base pairs consist of two species that differ by a single proton. In the context of buffers, the conjugate pair allows the solution to counteract changes in pH by donating or accepting a proton, which is fundamental to the buffering mechanism. The selection of an appropriate conjugate pair is critical for creating an effective buffer at the desired pH.

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation, pH = pKa + log([Base]/[Acid]), is used to relate the pH of a buffer solution to the ratio of the concentrations of the conjugate base to the acid, along with the acid’s dissociation constant (pKa). This relationship helps in designing buffer systems by indicating how the pH will shift as the ratio of components is changed, and it is especially useful when the buffer pH is close to the pKa of the weak acid.

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