Question

Buffered solutions are mixtures of a weak acid and its conjugate base. Explain why a mixture of a strong acid and its conjugate base (such as \(\mathrm{HCl}\) and \(\mathrm{Cl}^{-}\) ) is not buffered.

Short answer

A mixture of a strong acid, like HCl, and its conjugate base, such as Cl⁻, cannot form a buffered solution because the strong acid fully ionizes in water, releasing a large amount of H⁺ ions. The presence of the conjugate base is not enough to counteract the high concentration of H⁺ ions, and they lack the ability to effectively accept H⁺ ions due to the dominance of the strong acid in the equilibrium. Consequently, the pH will not remain relatively constant when small amounts of strong acids or bases are added, which is the main purpose of a buffer solution.

Step-by-step solution

Understand the properties of strong acids and weak acids

A strong acid, like HCl, completely ionizes in water, releasing a large amount of H⁺ ions. This means that the concentration of H⁺ ions in the solution is very high, leading to a low pH. A weak acid, in contrast, only partially ionizes in water, releasing a relatively smaller amount of H⁺ ions, and maintaining a higher pH compared to the same concentration of strong acid.

Recall the concept of a buffer solution

A buffer solution resists changes in pH upon the addition of small amounts of strong acids or bases. This is because the weak acid and its conjugate base can neutralize both added H⁺ and OH⁻ ions. The weak acid will donate H⁺ ions when the solution becomes more basic, while the conjugate base will accept H⁺ ions when the solution becomes more acidic. This balance helps maintain the pH of the solution within a relatively narrow range.

Explain why a strong acid and its conjugate base cannot form a buffer

In a solution containing a strong acid (HCl) and its conjugate base (Cl⁻), the strong acid will fully ionize, releasing a large amount of H⁺ ions. The presence of the Cl⁻ ions will not be enough to counteract the high concentration of H⁺ ions since the strong acid will dominate the dissociation equilibrium. When a base is added, the OH⁻ ions will react with the abundant H⁺ ions to form water, but the pH will not change significantly as the strong acid will continue to ionize, supplying more H⁺ ions. Similarly, when an acid is added, the solution will become more acidic, and the Cl⁻ ions will not be able to buffer it since they lack the ability to effectively accept H⁺ ions due to the dominance of the strong acid in the equilibrium.

Conclusion

A mixture of a strong acid and its conjugate base, such as HCl and Cl⁻, is not buffered because the strong acid fully ionizes in water, leading to a high concentration of H⁺ ions. The presence of the conjugate base is not enough to counteract this high concentration and maintain a relatively constant pH when small amounts of strong acids or bases are added.