Question

Which of the following mixtures would result in a buffered solution when 1.0 \(\mathrm{L}\) of each of the two solutions are mixed? $$ \begin{array}{l}{\text { a. } 0.2 M \mathrm{HNO}_{3} \text { and } 0.4 \mathrm{M} \mathrm{NaNO}_{3}} \\ {\text { b. } 0.2 \mathrm{M} \mathrm{HNO}_{3} \text { and } 0.4 \mathrm{M} \mathrm{HF}}\end{array} $$ $$ \begin{array}{l}{\text { c. } 0.2 M \mathrm{HNO}_{3} \text { and } 0.4 \mathrm{M} \mathrm{NaF}} \\ {\text { d. } 0.2 \mathrm{M} \mathrm{HNO}_{3} \text { and } 0.4 \mathrm{M} \mathrm{NaOH}}\end{array} $$

Short answer

The buffered solution is obtained when mixing 1.0 L of mixture b (0.2 M HNO$_{3}$ and 0.4 M HF) and 1.0 L of mixture c (0.2 M HNO$_{3}$ and 0.4 M NaF) as it combines a weak acid (HF) and its conjugate base (F-).

Step-by-step solution

Identify the strong acids and bases

In the given mixtures, some strong acids and bases, like HNO3 and NaOH, can be identified, which won't be part of a buffer system.

Identify weak acids and their conjugate bases

In the given mixtures, we can identify weak acids like HF (Hydrofluoric acid) along with their conjugate base like NaF (Sodium Fluoride). The conjugate base of HF is F- (Fluoride ion) and can be found in NaF.

Match the suitable weak acid and its conjugate base pair

Since mixture b has the weak acid HF and mixture c has its conjugate base NaF when these two solutions are mixed (1.0 L each), it will result in a buffered solution as the mixed solution will contain the weak acid and its conjugate base pair (HF and F-). So the combination of mixture b and mixture c will result in a buffered solution when mixed in equal volumes.

Key concepts

Acid-Base Chemistry

Acid-base chemistry is a fascinating branch of chemistry that explores the behavior of acids and bases in solutions. Acids are substances that can donate a proton (H⁺ ion) to another substance, while bases are substances that can accept a proton. This is often referred to as the Brønsted-Lowry definition. Understanding acid-base chemistry is crucial as it is the foundation for learning about buffering solutions, which are vital in maintaining the pH balance in various environments.

When acids and bases mix in solution, they can undergo reactions known as neutralization reactions, where the acid and base neutralize each other's properties, often resulting in the formation of water and a salt. However, this doesn't always result in neutral pH. The strength of the acid and base, whether they're strong or weak, can influence the outcome significantly. In studying these reactions, recognizing how different acids and bases behave helps determine when a buffered solution might form.

Buffered solutions are particularly important because they help maintain stable pH levels, which is essential in biological systems, chemical reactions, and many industrial applications.

Conjugate Acid-Base Pair

The concept of conjugate acid-base pairs is central to understanding acid-base reactions. When an acid donates a proton, it becomes its conjugate base, and when a base accepts a proton, it becomes its conjugate acid. This relationship forms a conjugate acid-base pair.

It's helpful to look at the example of hydrofluoric acid (HF). When HF donates a proton, it transforms into fluoride ion (F⁻), its conjugate base. Conversely, when F⁻ accepts a proton, it returns to HF, its conjugate acid. This dynamic equilibrium between HF and F⁻ allows for the solution to act as a buffer.

Understanding these pairs is crucial when studying buffers. Buffer solutions are typically composed of a weak acid along with its conjugate base. These solutions can resist changes in pH upon addition of small amounts of acids or bases. Therefore, identifying the right conjugate acid-base pair is essential when creating buffer solutions for chemical and biological applications.

Weak Acid and Conjugate Base Pairs

Weak acids and their conjugate base pairs form the backbone of many buffer solutions. Unlike strong acids, which fully dissociate in water, weak acids only partially dissociate, establishing an equilibrium system. This partial dissociation is key in forming buffers because it allows the solution to neutralize small quantities of added acid or base.

For a practical example, let's consider hydrofluoric acid (HF). HF is a weak acid because it doesn’t completely dissociate into its ions when dissolved in water. Its conjugate base, the fluoride ion (F⁻), is present in solutions like sodium fluoride (NaF). When HF and NaF are combined, they create a buffered system where both the weak acid and conjugate base are available to neutralize added acids or bases.

This buffering action works because the weak acid can donate protons to neutralize added bases, while the conjugate base can accept protons to neutralize added acids. Therefore, learning about weak acids and their conjugate bases is pivotal for understanding how buffers maintain stability in pH, especially in environments that require precise pH control, such as biological systems and chemical laboratories.