Question

Which substance could you add to each solution to make it a buffer solution? (a) \(0.050 \mathrm{M} \mathrm{NaHSO}_{3}\) (b) \(0.150 \mathrm{M} \mathrm{HF}\) (c) \(0.200 \mathrm{M} \mathrm{KCHO}_{2}\)

Short answer

To make buffer solutions add: (a) NaHSO3 or KHSO3, (b) NaF or KF, and (c) HCHO2.

Step-by-step solution

Understanding Buffers

A buffer solution is one that resists changes in pH when small amounts of acid or base are added. Most buffers consist of a weak acid and its conjugate base or a weak base and its conjugate acid.

Identifying the Conjugate Pair for NaHSO3

For solution (a), NaHSO3 is a weak acid (H2SO3 after losing one H+), and its conjugate base is HSO3-. To create a buffer, we need to add a salt containing the conjugate base, which could be NaHSO3 or KHSO3.

Identifying the Conjugate Pair for HF

For solution (b), HF is a weak acid and its conjugate base is F-. To create a buffer, you can add NaF or KF, which are salts that will provide the F- ion.

Identifying the Conjugate Pair for KCHO2

For solution (c), KCHO2 contains the formate ion (CHO2-), which is a weak base and its conjugate acid is HCHO2 (formic acid). To make a buffer, you can add HCHO2 to the solution.

Key concepts

Acid-Base Equilibrium

Balancing the scales of acid and base reactions is akin to a dance of protons between molecules. When an acid donates a proton (H+), it turns into its conjugate base, whereas when a base accepts a proton, it becomes its conjugate acid. This exchange is quintessential to understanding acid-base equilibrium. In any given solution, the acid and its conjugate base (or the base and its conjugate acid) exist in a dynamic balance. Leveraging the equilibrium constant, Ka for weak acids or Kb for weak bases, allows chemists to deduce the strengths of acid or base solutions and predict their behavior when mixed with other substances.

When delving into buffer solutions as in our exercise, the goal is to create conditions that minimize the shift in this careful balance when an acid or base is introduced. For example, in case (a), introducing a salt such as KHSO3 combines with NaHSO3 to maintain the equilibrium between H2SO3 (the weak acid) and HSO3- (the conjugate base), thus creating a buffer that can absorb fluctuations in pH levels.

Conjugate Acid-Base Pairs

To grasp the nuances of buffer solutions, one must first become familiar with conjugate acid-base pairs. These are two species that transform into each other by the gain or loss of a proton. Returning to our textbook solutions, for substance (b), we observe that HF (hydrofluoric acid), upon releasing a proton, becomes F- (fluoride ion), its conjugate base. Conversely, if F- were to gain a proton, it would revert back to HF, showcasing the reversible nature of the acid-base relationship.

In the context of buffers, adding a compound containing the conjugate base, such as NaF for part (b), ensures that whenever additional acid arrives into the scene, the F- ions are ready to pair up with the incoming H+ ions, maintaining the stability of the solution's pH. This interplay is critical for processes such as biochemical reactions in the human body, where pH fluctuations can have significant consequences.

Resisting pH Change

The raison d'etre for buffer solutions is their ability to ward off pH changes when faced with the addition of acids or bases. This resistance is a product of the acid-base equilibrium we've previously discussed, and the presence of conjugate acid-base pairs. Part (c) of our exercise explores KCHO2, a salt, that when combined with HCHO2 (formic acid), creates such a buffer system involving the formate ion (CHO2-) and its conjugate acid HCHO2.

The concoction operates on the principle of Le Châtelier, where the equilibrium shifts to counteract the change introduced by the added acid or base. By supplementing the solution with either member of the conjugate pair, we provide a means for either neutralizing extra H+ ions (using the conjugate base) or providing H+ ions when a base is added (using the conjugate acid). Through this equilibrium response, the pH of the solution remains relatively stable, which is quintessential in many biological and industrial applications.