Problem 52 - Chemistry: Structure and Properties

Question

Determine whether the mixing of each pair of solutions results in a buffer. a. 75.0 mL of 0.10 M HF; 55.0 mL of 0.15 M NaF b. 150.0 mL of 0.10 M HF; 135.0 mL of 0.175 M HCl c. 165.0 mL of 0.10 M HF; 135.0 mL of 0.050 M KOH d. 125.0 mL of 0.15 M CH3NH2; 120.0 mL of 0.25 M CH3NH3Cl e. 105.0 mL of 0.15 M CH3NH2; 95.0 mL of 0.10 M HCl

Step-by-Step Solution

Answer

a. Yes, b. No, c. No, d. Yes, e. No. Only the mixtures that include a weak acid/base and its conjugate base/acid produce buffer solutions.

Step 1: Understand Buffer Solutions

A buffer solution consists of a weak acid and its conjugate base or a weak base and its conjugate acid. The mixture is able to neutralize added acid or base, thereby maintaining the pH level relatively constant.

Step 2: Evaluate Pair a (HF and NaF)

HF is a weak acid and NaF provides the conjugate base F-. When these two are mixed, they form a buffer solution because they consist of a weak acid and its conjugate base in solution.

Step 3: Evaluate Pair b (HF and HCl)

HF is a weak acid, while HCl is a strong acid. Mixing these two will not form a buffer because HCl will dissociate completely, and there is no conjugate base present to pair with HF.

Step 4: Evaluate Pair c (HF and KOH)

HF is a weak acid and KOH is a strong base. The KOH will react with HF to form water and the salt KF. This reaction does not produce a buffer because the strong base will neutralize the weak acid.

Step 5: Evaluate Pair d (CH3NH2 and CH3NH3Cl)

CH3NH2 is a weak base and CH3NH3Cl is its conjugate acid (CH3NH3+). When mixed, they form a buffer solution composed of a weak base and its conjugate acid.

Step 6: Evaluate Pair e (CH3NH2 and HCl)

CH3NH2 is a weak base and HCl is a strong acid. The strong acid will react with the weak base to form the salt CH3NH3Cl and water. Although a conjugate acid is formed, the HCl will not be present as a weak conjugate base is needed for a buffer, thus not forming a buffer solution.

Key Concepts

Weak Acid and Conjugate Base

A weak acid is an acid that does not completely dissociate in solution, meaning it does not release all of its hydrogen ions into the solution. The conjugate base is the species that remains after the weak acid has donated a hydrogen ion. Together, they create a push-and-pull equilibrium that can absorb fluctuations in pH levels.

For example, when hydrofluoric acid (HF) is dissolved in water, it partially ionizes to form H⁺ ions and fluoride ions (F^-). The fluoride ions are the conjugate base of the weak acid HF. When mixed with HF, a source of the conjugate base such as sodium fluoride (NaF), which provides the F^- ions in solution, they create a buffer solution capable of stabilizing the pH upon the addition of other acids or bases.

Weak Base and Conjugate Acid

A weak base is similar to a weak acid but in reverse; it accepts hydrogen ions from water to form its conjugate acid. The weak base and its conjugate acid work in conjunction to prevent large swings in pH.

For instance, methylamine (CH₃NH₂) reacts with water to form methylammonium ions (CH₃NH₃⁺) and hydroxide ions (OH^-). Methylammonium ions are the conjugate acid of the weak base methylamine. When combined with an appropriate amount of its conjugate acid, like in the case of CH₃NH₂ mixed with methylammonium chloride (CH₃NH₃Cl), it forms a buffering system that neutralizes additional acids or bases, maintaining the solution's pH.

Neutralization Reaction

Neutralization is the chemical reaction in which an acid and a base react quantitatively with each other. In a neutralization reaction, the acid and the base generally form a salt and water, which often results in the pH moving toward 7, or becoming neutral.

However, if a strong acid is mixed with a weak base, or vice versa, the resulting pH will not be neutral due to the complete dissociation of the strong acid or base. For example, the neutralization of HCl, a strong acid, with CH₃NH₂, a weak base, forms the salt CH₃NH₃Cl and water, but the pH will still be on the acidic side due to the complete ionization of HCl.

pH Stabilization

pH stabilization is key in buffer solutions, which are designed to resist changes in pH when small amounts of acid or base are added. Buffers work according to Le Chatelier's Principle; when the system is disturbed, it will adjust to minimize the change.

This is achieved by the equilibrium that exists between the weak acid and its conjugate base, or the weak base and its conjugate acid. When an acid is added, the conjugate base will react with the added H⁺ ions, minimizing the pH change. Conversely, if a base is added, the weak acid will donate H⁺ ions to combine with the excess OH^- ions, also minimizing the pH change. This dynamic process provides the pH stabilization essential in many biological and chemical applications.