A student is asked to prepare a buffer solution at pH 8.60,...

A student is asked to prepare a buffer solution at pH $8.60,$ using one of the following weak acids: $\mathrm{HA}\left(K_{a}=2.7 \times 10^{-3}\right), \mathrm{HB}$ $\left(K_{\mathrm{a}}=4.4 \times 10^{-6}\right), \mathrm{HC}\left(K_{\mathrm{a}}=2.6 \times 10^{-9}\right) .$ Which acid should the student choose? Why?

Key Concepts

Buffer Solution

A buffer solution is a mixture of a weak acid and its conjugate base (or a weak base and its conjugate acid) that resists changes in pH upon the addition of small quantities of acids or bases. It works by neutralizing added acids through the base component and neutralizing added bases through the acid component, thereby maintaining the overall pH of the solution within a narrow range.

pH is a measure of the hydrogen ion concentration in a solution, indicating its acidity or basicity. It is defined on a logarithmic scale where a lower pH represents a higher concentration of hydrogen ions (more acidic) and a higher pH represents a lower concentration (more basic). The target pH for a buffer is crucial for ensuring the buffer operates optimally in the desired range.

Acid Dissociation Constant (Ka) and pKa

The acid dissociation constant, Ka, quantifies the strength of a weak acid by indicating its tendency to donate protons to water. The pKa, which is the negative logarithm of the Ka, is a convenient value that helps in comparing acid strengths: a lower pKa indicates a stronger acid. In buffer preparation, choosing an acid with a pKa close to the desired pH ensures effective buffering capacity.

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation is used to relate the pH of a buffer solution to the pKa of the acid and the ratio of the concentrations of the conjugate base to the weak acid. This equation is crucial for designing buffer solutions, as it provides a straightforward method to calculate the necessary proportions of the acid and its conjugate base to achieve a specific pH.

Buffer Selection Based on pKa Proximity

When selecting a weak acid for buffer preparation, it is important to choose one whose pKa is near the target pH. This proximity ensures that the concentrations of the acid and its conjugate base are comparable, providing optimal capacity to neutralize added acids or bases. This selection criterion is essential because if the pKa is too far from the desired pH, the buffer will be less effective in stabilizing the pH of the solution.

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