Question

A best buffer has about equal quantities of weak acid and conjugate base present as well as having a large concentration of each species present. Explain.

Short answer

A best buffer has about equal quantities of weak acid and conjugate base present because it maximizes its ability to neutralize added acids and bases, maintaining the pH at the \(pK_a\) value of the weak acid. Having a large concentration of each species present increases the buffering capacity, making the solution more resistant to pH changes due to Le Chatelier's principle. This ensures that the buffer can effectively maintain the desired pH range when reacting with added acidic or basic substances.

Step-by-step solution

Understanding Buffers

A buffer solution is a solution that resists changes in pH when small amounts of acid or base are added. They maintain the equilibrium of acidic and basic components in a solution. Buffers consist of a weak acid and its conjugate base or a weak base and its conjugate acid.

Henderson-Hasselbalch Equation

The pH of a buffer solution can be described by the Henderson-Hasselbalch equation: \[pH = pK_a + \log_{10} \frac{[A^-]}{[HA]}\] where \(pK_a\) is the negative logarithm of the acid dissociation constant (\(K_a\)), \([A^-]\) is the molar concentration of the conjugate base, and \([HA]\) is the molar concentration of the undissociated weak acid. The equation shows the relationship between the pH, the concentrations of the acid and conjugate base, and the \(pK_a\).

Equal Quantities of Weak Acid and Conjugate Base

When the concentrations of the weak acid (\([HA]\)) and its conjugate base (\([A^-]\)) are equal, the term in the logarithm becomes: \[\log_{10} \frac{[A^-]}{[HA]} = \log_{10} 1 = 0\] So the pH in this situation is equal to the \(pK_a\), which means that the buffer is most effective at maintaining the pH at the \(pK_a\) value of the weak acid. In other words, the buffer has the highest capacity to neutralize both added acids and bases when \([A^-] = [HA]\).

Large Concentration of Species

The larger the concentrations of both weak acid and conjugate base in the buffer solution, the greater its buffering capacity. With higher concentrations, more acid or base can be added to the solution before the pH starts to change significantly. This is due to Le Chatelier's principle: the system will shift the equilibrium position to minimize the effect of a change in concentration. So, if the buffer has a large amount of both weak acid and conjugate base, it is more capable of maintaining the pH in a desired range and resisting changes caused by the addition of other acidic or basic substances. To conclude, a best buffer has about equal quantities of weak acid and conjugate base present, which maximizes its ability to neutralize added acids and bases, and having a large concentration of each species present increases the buffering capacity, making the solution more resistant to pH changes.