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Chapter 15: Problem 69

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

Expert verified
A mixture of a strong acid, like HCl, and its conjugate base, such as Cl鈦�, cannot form a buffered solution because the strong acid fully ionizes in water, releasing a large amount of H鈦� ions. The presence of the conjugate base is not enough to counteract the high concentration of H鈦� ions, and they lack the ability to effectively accept H鈦� 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

01

Understand the properties of strong acids and weak acids

A strong acid, like HCl, completely ionizes in water, releasing a large amount of H鈦� ions. This means that the concentration of H鈦� 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鈦� ions, and maintaining a higher pH compared to the same concentration of strong acid.
02

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鈦� and OH鈦� ions. The weak acid will donate H鈦� ions when the solution becomes more basic, while the conjugate base will accept H鈦� ions when the solution becomes more acidic. This balance helps maintain the pH of the solution within a relatively narrow range.
03

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鈦�), the strong acid will fully ionize, releasing a large amount of H鈦� ions. The presence of the Cl鈦� ions will not be enough to counteract the high concentration of H鈦� ions since the strong acid will dominate the dissociation equilibrium. When a base is added, the OH鈦� ions will react with the abundant H鈦� ions to form water, but the pH will not change significantly as the strong acid will continue to ionize, supplying more H鈦� ions. Similarly, when an acid is added, the solution will become more acidic, and the Cl鈦� ions will not be able to buffer it since they lack the ability to effectively accept H鈦� ions due to the dominance of the strong acid in the equilibrium.
04

Conclusion

A mixture of a strong acid and its conjugate base, such as HCl and Cl鈦�, is not buffered because the strong acid fully ionizes in water, leading to a high concentration of H鈦� 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.

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Most popular questions from this chapter

Which of the following bases have relatively strong conjugate acids? a. \(\mathrm{F}^{-}\) b. \(\mathrm{Cl}^{-}\) c. \(\mathrm{HSO}_{4}^{-}\) d. \(\mathrm{NO}_{3}^{-}\)

An acid such as \(\mathrm{HCl}\) that strongly conducts an electric current when dissolved in water is said to be \(a(n)\) _____ acid.

For each pair of concentrations, tell which represents the more basic solution. a. \(\left[\mathrm{H}^{+}\right]=1.59 \times 10^{-7} \mathrm{M}\) or \(\left[\mathrm{H}^{+}\right]=1.04 \times 10^{-8} \mathrm{M}\) b. \(\left[\mathrm{H}^{+}\right]=5.69 \times 10^{-8} \mathrm{M}\) or \(\left[\mathrm{OH}^{-}\right]=4.49 \times 10^{-6} \mathrm{M}\) c. \(\left[\mathrm{H}^{+}\right]=5.99 \times 10^{-8} \mathrm{M}\) or \(\left[\mathrm{OH}^{-}\right]=6.01 \times 10^{-7} \mathrm{M}\)

For each pair of concentrations, tell which represents the more acidic solution. a. \(\left[\mathrm{H}^{+}\right]=1.2 \times 10^{-3} \mathrm{M}\) or \(\left[\mathrm{H}^{+}\right]=4.5 \times 10^{-4} \mathrm{M}\) b. \(\left[\mathrm{H}^{+}\right]=2.6 \times 10^{-6} \mathrm{M}\) or \(\left[\mathrm{H}^{+}\right]=4.3 \times 10^{-8} \mathrm{M}\) c. \(\left[\mathrm{H}^{+}\right]=0.000010 \mathrm{M}\) or \(\left[\mathrm{H}^{+}\right]=0.0000010 \mathrm{M}\)

Calculate the \(\left[\mathrm{OH}^{-}\right]\) in each of the following solutions, and indicate whether the solution is acidic or basic. a. \(\left[\mathrm{H}^{+}\right]=1.34 \times 10^{-2} \mathrm{M}\) b. \(\left[\mathrm{H}^{+}\right]=6.99 \times 10^{-7} \mathrm{M}\) c. \(\left[\mathrm{H}^{+}\right]=4.01 \times 10^{-9} \mathrm{M}\) d. \(\left[\mathrm{H}^{+}\right]=4.02 \times 10^{-13} \mathrm{M}\)
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