Question

Which of the following salt solutions is highly acidic?

• A. Ammonium acetate
• B. Ammonium cyanide
• C. Sodium chloride
• D. Ammonium nitrate

Hint:

To quickly find the $\mathrm{pH}$ of a salt solution, split the salt into its parent components. The stronger parent always dominates the final properties of the solution. Since Ammonium Nitrate comes from Nitric Acid (Strong) and Ammonium Hydroxide (Weak), the acid wins, yielding an acidic solution!

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
We are given four different chemical salts and need to evaluate their chemical compositions to determine which one produces a highly acidic aqueous solution upon dissolving in water.

Step 2: Key Formula or Approach:
The $\mathrm{pH}$ behavior of a salt solution is governed by salt hydrolysis. The specific acid-base nature of the parent reagents determines the final state of the solution: 1. Strong Acid + Strong Base $\rightarrow$ Neutral solution ($\mathrm{pH} = 7$) 2. Strong Acid + Weak Base $\rightarrow$ Acidic solution ($\mathrm{pH} < 7$) due to cationic hydrolysis. 3. Weak Acid + Strong Base $\rightarrow$ Basic solution ($\mathrm{pH} > 7$) due to anionic hydrolysis.

Step 3: Detailed Explanation:
Let's trace the parent acids and bases for each option: Ammonium acetate ($\mathrm{CH_3COONH_4}$): Derived from a weak acid ($\mathrm{CH_3COOH}$) and a weak base ($\mathrm{NH_4OH}$). Since their dissociation constants are nearly identical ($K_a \approx K_b$), the solution remains roughly neutral. Ammonium cyanide ($\mathrm{NH_4CN}$): Derived from a weak base ($\mathrm{NH_4OH}$) and a very weak acid ($\mathrm{HCN}$). Because $K_b > K_a$, this solution is slightly basic. Sodium chloride ($\mathrm{NaCl}$): Derived from a strong base ($\mathrm{NaOH}$) and a strong acid ($\mathrm{HCl}$). Neither ion undergoes hydrolysis, so the solution is completely neutral ($\mathrm{pH} = 7$). Ammonium nitrate ($\mathrm{NH_4NO_3}$): Derived from the combination of a weak base ($\mathrm{NH_4OH}$) and a strong mineral acid ($\mathrm{HNO_3}$). In water, the ammonium cation ($\mathrm{NH_4^+}$) undergoes cationic hydrolysis, donating protons to water molecules and releasing excess hydronium ions into solution: $$\mathrm{NH_4^+} + \mathrm{H_2O} \rightleftharpoons \mathrm{NH_4OH} + \mathrm{H^+}$$ Because the nitrate ion ($\mathrm{NO_3^-}$) is the conjugate partner of a strong acid, it does not hydrolyze. This leaves a significant excess of free $\mathrm{H^+}$ ions, making the solution highly acidic.

Step 4: Final Answer:
The salt solution that is highly acidic is Ammonium nitrate, matching option (D).