Question

Does the \(\mathrm{pH}\) of the solution increase, decrease, or stay the same when you (a) add solid sodium oxalate, \(\mathrm{Na}_{2} \mathrm{C}_{2} \mathrm{O}_{4},\) to \(50.0 \mathrm{mL}\) of \(0.015 \mathrm{M}\) oxalic acid, \(\mathrm{H}_{2} \mathrm{C}_{2} \mathrm{O}_{4} ?\) (b) add solid ammonium chloride to \(75 \mathrm{mL}\) of \(0.016 \mathrm{M} \mathrm{HCl} ?\) (c) add \(20.0 \mathrm{g}\) of \(\mathrm{NaCl}\) to \(1.0 \mathrm{L}\) of \(0.10 \mathrm{M}\) sodium acetate, \(\mathrm{NaCH}_{3} \mathrm{CO}_{2} ?\)

Short answer

(a) Increases; (b) Stays the same; (c) Stays the same.

Step-by-step solution

Understanding the Chemical Solutions

First, identify the components involved in each part of the question. Oxalic acid is a weak acid, ammonium chloride is a salt of a weak base (ammonia) and HCl, and sodium acetate is a salt of a weak acid (acetic acid).

Analyzing Part (a) - Sodium Oxalate and Oxalic Acid

Sodium oxalate, when added to oxalic acid, introduces the oxalate ion, which is the conjugate base of oxalic acid. This sets up a buffer system, stabilizing the pH. Generally, the addition of a salt containing the conjugate base of a weak acid results in an increase in pH due to the common ion effect.

Analyzing Part (b) - Ammonium Chloride and HCl

Adding ammonium chloride to an HCl solution increases the amount of the ammonium ion. Since both HCl and NH4+ are acidic, the solution remains acidic, and there is no buffering system to significantly change the pH. Thus, the pH remains relatively the same.

Analyzing Part (c) - Sodium Chloride and Sodium Acetate

Sodium chloride is a neutral salt and does not affect the pH. Sodium acetate in solution forms a basic buffer. Adding NaCl does not significantly interact with this buffer system, so the pH remains the same.

Key concepts

Buffer Solutions

Buffers are solutions that resist dramatic changes in pH when small amounts of an acid or base are added to them. This stability is due to the presence of a weak acid and its conjugate base, or a weak base and its conjugate acid, in roughly equal concentrations. When an acid is added to a buffer, the conjugate base present in the buffer reacts with the acid, neutralizing it and minimizing the change in pH. Similarly, when a base is added, the weak acid in the buffer neutralizes the base.
In step 2 of the solution, a buffer is established when sodium oxalate is added to oxalic acid, creating a combination of a weak acid and its conjugate base. Thus, the pH increases slightly due to the common ion effect.

Common Ion Effect

The common ion effect refers to the phenomenon where the addition of an ion common to the equilibrium existing in a solution shifts the equilibrium position. This shift occurs according to Le Chatelier's Principle, which aims to re-establish equilibrium by opposing the change.
In the exercise, sodium oxalate introduces more oxalate ions into an oxalic acid solution. The presence of these additional oxalate ions causes the equilibrium to shift, reducing the concentration of hydrogen ions and increasing the pH.

• This adjustment of pH is a part of buffer systems responding to added common ions.
• It demonstrates how conjugate base pairs interact to stabilize solutions.

Conjugate Acid-Base Pairs

Conjugate acid-base pairs consist of molecules or ions that are related by the gain or loss of a proton ( ext{H}^+) in a reversible chemical reaction. The acid member of the pair donates a proton, while the base member accepts a proton.
In the original solution, oxalic acid, ext{H}_2 ext{C}_2 ext{O}_4, loses a proton to become the oxalate ion, ext{C}_2 ext{O}_4^{2-}, when sodium oxalate is added.

• These pairs are crucial in buffer solutions where they work to neutralize added acids or bases.
• Recognizing conjugate acid-base pairs help in understanding how buffers maintain pH balance.

Weak Acids and Bases

Weak acids and bases do not completely ionize in solution, meaning they establish an equilibrium between the ionized and unionized forms. This characteristic makes them suitable for forming buffer solutions.
Oxalic acid is a weak acid, while acetic acid, present in sodium acetate, is also a weak acid.

• Their partial ionization allows them to participate effectively in buffer systems.
• This property enables buffers to resist changes in pH upon the addition of small quantities of strong acids or bases.

Understanding the behavior of weak acids and bases aids in predicting the pH behavior of different chemical solutions.