Which of the following ions will interact with water to give a solution of pH

Question

Which of the following ions will interact with water to give a solution of pH < 7? A.Na+ B.NH4+ C.CN- D.HCOO-

ScanSolve AI Tutor · Accepted Answer

To determine which ion will give a solution with pH < 7 (acidic), we need to consider the hydrolysis of each ion in water.

Analyze the nature of each ion.
- A. Na $^+$ : This is the conjugate acid of a strong base (NaOH). Ions from strong bases do not hydrolyze significantly in water, so they do not affect the pH.
- B. NH $_4^+$ : This is the conjugate acid of a weak base (NH $_3$ ). Conjugate acids of weak bases are acidic and will react with water to produce H $_3$ O $^+$ ions.
- C. CN $^-$ : This is the conjugate base of a weak acid (HCN). Conjugate bases of weak acids are basic and will react with water to produce OH $^-$ ions.
- D. HCOO $^-$ : This is the conjugate base of a weak acid (formic acid, HCOOH). Similar to CN $^-$ , it will react with water to produce OH $^-$ ions.
Write the hydrolysis reaction for the ion that would produce an acidic solution.
An acidic solution has a pH less than 7, meaning it has an excess of H $_3$ O $^+$ ions. From Step 1, NH $_4^+$ is the only ion that is an acidic conjugate.
The hydrolysis reaction for NH $_4^+$ is:
$\text{NH}_4^+ (aq) + \text{H}_2\text{O} (l) \rightleftharpoons \text{NH}_3 (aq) + \text{H}_3\text{O}^+ (aq)$
This reaction produces hydronium ions ( $\text{H}_3\text{O}^+$ ), which makes the solution acidic, resulting in a pH less than 7.

The other ions would either not affect the pH (Na $^+$ ) or make the solution basic (CN $^-$ and HCOO $^-$ ).

The correct option is B.

$\boxed{\text{B. NH}_4^+}$

Which of the following ions will interact with water to give a solution of pH < 7? A.Na+ B.NH4+ C.CN- D.HCOO-