Problem 37 - Chemistry in 30 days

Question

In which of following salts, \(\mathrm{pH}\) is not affected by dilution ? (a) \(\mathrm{CuCl}_{2}\) (b) \(\mathrm{CH}_{3} \mathrm{COONa}\) (c) \(\mathrm{CH}_{3} \mathrm{COONH}_{4}\) (d) \(\mathrm{NH}_{4} \mathrm{Cl}\)

Step-by-Step Solution

Answer

(c) \(\mathrm{CH_{3}COONH_{4}}\), as its pH is minimally affected by dilution.

Step 1: Understand the Problem

The exercise asks to identify which salt's pH is not affected by dilution. Understanding that dilution affects the pH of weak acid and weak base salts, while strong acid-strong base salts have constant pH regardless of dilution, is crucial.

Step 2: Identify the Types of Salts

Review each salt in the list: - \(\mathrm{CuCl_{2}}\) is from a strong acid (HCl) and weak base (Cu(OH)\(_2\)).- \(\mathrm{CH_{3}COONa}\) is from a weak acid (CH\(_3\)COOH) and a strong base (NaOH).- \(\mathrm{CH_{3}COONH_{4}}\) is from a weak acid (CH\(_3\)COOH) and a weak base (NH\(_3\)).- \(\mathrm{NH_{4}Cl}\) is from a strong acid (HCl) and a weak base (NH\(_3\)).

Step 3: Consider the Effect of Dilution

Dilution generally affects the pH of solutions created from a weak acid or weak base. Therefore, salts formed from strong acids and strong bases are not strongly affected by dilution in terms of pH.

Step 4: Identify the Correct Answer

Among the options, the only salt formed from a strong acid and a strong base mixture, which results in minimal pH change upon dilution, is \(\mathrm{CH_{3}COONH_{4}}\) because it is derived from weak acid and weak base but in this case it can remain relatively stable.

Key Concepts

Salt Solutions

Salt solutions are created when a salt, the product of a chemical reaction between an acid and a base, dissolves in water. These solutions can have varying pH levels depending on the nature of the acid and base involved. If a strong acid reacts with a strong base, the salt solution tends to be neutral, having a pH around 7.

• Strong Acid + Strong Base: Neutral solution (e.g., NaCl).
• Strong Acid + Weak Base: Acidic solution (e.g., NH extsubscript{4}Cl).
• Weak Acid + Strong Base: Basic solution (e.g., CH extsubscript{3}COONa).
• Weak Acid + Weak Base: pH can vary and is influenced by the relative strengths of the acid and base (e.g., CH extsubscript{3}COONH extsubscript{4}).

Different combinations can lead to acidic, basic, or neutral solutions. Understanding these interactions is key to predicting the behavior of salt solutions in various conditions. This knowledge is essential for determining how changes, such as dilution, might impact the pH of a solution.

Acid-Base Chemistry

Acid-base chemistry revolves around the properties and interactions of acids and bases. An acid is a substance that can donate a proton (H extsuperscript{+}), while a base is a substance that can accept a proton. When acids and bases react, they form a salt and water, a process called neutralization. The pH scale measures the acidity or basicity of a solution.

• The pH scale ranges from 0 to 14.
• Acids have a pH less than 7.
• Bases have a pH greater than 7.
• A pH of 7 is neutral.

Predicting how pH might change with dilution requires an understanding of the acid or base strength in a solution. Strong acids and bases fully dissociate in water, meaning they break apart completely to release their ions. This is why the pH of solutions formed from strong acids and strong bases is less sensitive to dilution. Weak acids and bases only partially dissociate, so their pH is often more susceptible to changes in concentration, such as those caused by dilution.

Chemical Dilution Effects

The process of dilution involves adding more solvent (usually water) to a solution, thereby reducing the concentration of solutes within it. This has specific effects on chemical solutions, especially those involving weak acids or bases. Here’s how dilution influences pH: - **Strong Acid + Strong Base Salts**: These remain neutral or exhibit a consistent pH upon dilution because their ions don't significantly affect the pH, as they do not react with water. - **Weak Acid or Weak Base Salts**: These solutions tend to experience a change in pH upon dilution. This is due to the small degree of ion dissociation in weak acids and bases, which is amplified when the concentration changes. For example, if you dilute a solution of NH extsubscript{4}Cl, the concentration of NH extsubscript{4} extsuperscript{+} and Cl extsuperscript{-} ions decreases. Since NH extsubscript{4} extsuperscript{+} is a weak acid, the pH might increase slightly with dilution. On the other hand, buffering occurs when dilution has little to no effect on pH. Salts from both weak acids and weak bases, such as CH extsubscript{3}COONH extsubscript{4}, may behave like this, maintaining a relatively stable pH even when diluted, due to the balanced ion interactions. Understanding these principles helps in predicting and controlling pH in various scientific and industrial applications.