Problem 98 - Chemistry The Molecular Science

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-M oxalic acid? (b) Add solid ammonium chloride to \(100 . \mathrm{mL}\) of \(0.016-\mathrm{M} \mathrm{HCl} ?\) (c) Add \(20.0 \mathrm{~g} \mathrm{NaCl}\) to \(1.0 \mathrm{~L}\) of \(0.012-\mathrm{M}\) sodium acetate, \(\mathrm{NaCH}_{3} \mathrm{COO} ?\)

Step-by-Step Solution

Answer

(a) pH increases; (b) pH stays the same; (c) pH stays the same.

Step 1: Analyze the Effect of Adding Na2C2O4 to Oxalic Acid

Adding sodium oxalate, \(\text{Na}_2\text{C}_2\text{O}_4\), introduces \(\text{C}_2\text{O}_4^{2-}\) ions into the solution. These ions are the conjugate base of the oxalic acid's second dissociation step, \(\text{HC}_2\text{O}_4^- \rightarrow \text{H}^+ + \text{C}_2\text{O}_4^{2-}\). The introduction of \(\text{C}_2\text{O}_4^{2-}\) shifts the equilibrium to the left, consuming \(\text{H}^+\) ions and increasing the \(\mathrm{pH}\) of the solution.

Step 2: Examine the Effect of Adding NH4Cl to HCl Solution

Ammonium chloride, \(\text{NH}_4\text{Cl}\), adds \(\text{NH}_4^+\), a weak acid, to the \(\text{HCl}\) solution. However, \(\text{HCl}\) is a strong acid and dominates the acidity of the solution. The added \(\text{NH}_4^+\) will not significantly change the \(\mathrm{pH}\) since the \(\text{HCl}\) already provides a high concentration of \(\text{H}^+\) ions. Therefore, the \(\mathrm{pH}\) remains the same.

Step 3: Assess the Effect of Adding NaCl to Sodium Acetate

When sodium chloride, \(\text{NaCl}\), is added to the sodium acetate solution, \(\text{NaCH}_3\text{COO}\), the primary products are sodium ions and chloride ions, both of which are spectators in terms of acid-base reactions. There is no change in the acetate ion concentration or the balance of acetic acid and acetate ions in the buffer solution. Thus, the \(\mathrm{pH}\) of the solution remains the same.

Key Concepts

pH Changes

Understanding how pH changes is crucial in chemistry, particularly when dealing with mixtures or solutions. The pH of a solution is a measure of its hydrogen ion concentration, \([ ext{H}^+] ext{,}\) with lower pH indicating higher acidity. Changes in pH occur when substances that can either donate or accept \( ext{H}^+\) ions are added to the solution.For instance, adding sodium oxalate introduces oxalate ions, which can accept hydrogen ions, effectively reducing the \([ ext{H}^+] ext{)}\) and resulting in an increase in pH. Conversely, adding a substance that increases \([ ext{H}^+] ext{)}\) will lower the pH, making the solution more acidic. However, not all additions impact pH; some substances, like the \( ext{NH}_4^+ ext{)}\) ion in the presence of strong acids like HCl, may have negligible effects due to already high hydrogen ion concentrations.Through careful analysis, predicting pH changes involves recognizing the nature of the added substances and their impact on the ion equilibrium within the solution.

Buffer Solutions

Buffer solutions play a vital role in maintaining pH stability within a solution. These solutions are typically composed of a weak acid and its conjugate base or a weak base and its conjugate acid. The unique feature of buffers is their ability to resist drastic pH changes, even when small amounts of acids or bases are added. When sodium oxalate, a conjugate base, is added to an oxalic acid solution, it serves as a classic example of a buffer system at play. The addition of oxalate ions shifts the equilibrium, counteracting any potential pH changes by neutralizing excess hydrogen ions via the conjugate base. This buffering action results in a relatively stable pH. On the other hand, when substances like NaCl are added to a sodium acetate solution, they do not affect the buffer components. Sodium and chloride ions are considered spectators, thus the buffer potential remains unchanged, and the pH remains stable.

Acid-Base Equilibrium

The concept of acid-base equilibrium is fundamental to understanding the behavior of solutions under various conditions. This involves the balance between acids and bases in a solution, particularly how weak acids and their conjugates can establish an equilibrium.Oxalic acid, for example, undergoes dissociation in steps, creating a delicate balance between itself, its conjugate base, and \( ext{H}^+\) ions. Introducing more \( ext{C}_2 ext{O}_4^{2-}\) ions from sodium oxalate shifts this equilibrium by consuming hydrogen ions, affecting the acid-base balance and increasing pH.Acid-base equilibrium extends to scenarios like adding ammonium chloride to HCl. Ammonium ions, being a weak acid, have minimal impact on the strong acidic presence of HCl, which firmly dictates the equilibrium and overall solution acidity.Evaluating acid-base equilibrium helps predict changes and conditions within a solution by understanding the roles of involved species and their interactions.