Arrange the following in increasing order of pH. KNO3 (aq), CH3COONa (aq), NH4Cl (aq), C6H5COONH4 (aq)

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Q. Arrange the following in increasing order of pH. KNO3 (aq), CH3COONa (aq), NH4Cl (aq), C6H5COONH4 (aq)

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Concept understanding — Brønsted Lowry Conjugate Pairs

From Intuition to Precision: Brønsted–Lowry Conjugate Pairs

Imagine you're at a party where people are passing around a single, very special coin. The coin represents a proton ( $\text{H}^+$ ). The game is simple: someone can give the coin to someone else, but only if the other person is willing to take it. You can't just throw the coin; you need a willing receiver.

In chemistry, acids and bases play exactly this game. An acid is the person who gives away the proton. A base is the person who accepts it. But here's the twist: the moment the acid gives away its proton, it transforms into something new — something that can now accept a proton back. That transformed form is called the conjugate base. Similarly, the base, after accepting the proton, becomes something that can donate it back — the conjugate acid.

This is the core of the Brønsted–Lowry theory: every acid-base reaction involves a pair of substances that are linked by the gain or loss of a single proton.

The Precise Statement

\text{Acid} \rightleftharpoons \text{Conjugate Base} + \text{H}^+

A conjugate acid-base pair consists of two species that differ by exactly one proton ( $\text{H}^+$ ). The acid has the proton; the conjugate base does not.

When an acid donates a proton, it becomes its conjugate base.

When a base accepts a proton, it becomes its conjugate acid.

Important

In any Brønsted–Lowry reaction, there are two conjugate pairs: one on the reactant side and one on the product side. They always appear together.

Seeing It in Action

Take the classic reaction between hydrochloric acid and water:

\text{HCl} + \text{H}_2\text{O} \rightarrow \text{Cl}^- + \text{H}_3\text{O}^+

Let's identify the pairs:

Pair 1: $\text{HCl}$ (acid) and $\text{Cl}^-$ (conjugate base). They differ by one $\text{H}^+$ .
Pair 2: $\text{H}_2\text{O}$ (base) and $\text{H}_3\text{O}^+$ (conjugate acid). They also differ by one $\text{H}^+$ .

Notice: water acted as a base here — it accepted the proton from HCl. But water can also act as an acid in other reactions. That's the beauty of the Brønsted–Lowry theory: a substance's role depends on the reaction, not on a fixed label.

Tip

To find the conjugate base of any acid, simply remove one $\text{H}^+$ and reduce the charge by +1.

To find the conjugate acid of any base, add one $\text{H}^+$ and increase the charge by +1.

A Quick Reference Table

Acid	Conjugate Base	Base	Conjugate Acid
$\text{HCl}$	$\text{Cl}^-$	$\text{NH}_3$	$\text{NH}_4^+$
$\text{H}_2\text{SO}_4$	$\text{HSO}_4^-$	$\text{H}_2\text{O}$	$\text{H}_3\text{O}^+$
$\text{NH}_4^+$	$\text{NH}_3$	$\text{OH}^-$	$\text{H}_2\text{O}$
$\text{H}_2\text{O}$	$\text{OH}^-$	$\text{CO}_3^{2-}$	$\text{HCO}_3^-$

Notice how water appears in both columns — it's amphoteric, meaning it can act as either an acid or a base depending on its partner.

Why This Matters …

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The pH of aqueous salt solutions depends on the hydrolysis of their constituent ions. By identifying the parent acid and base for each salt, we can determine if the solution is acidic, basic, or neutral. The increasing order of pH is NH $_4$ Cl < C $_6$ H $_5$ COONH $_4$ < KNO $_3$ < CH $_3$ COONa.

When a salt dissolves in water, its constituent ions may react with water molecules in a process called hydrolysis. This reaction can produce H $_3$ O $^+$ or OH $^-$ ions, thereby changing the pH of the solution from neutral. The extent of hydrolysis, and thus the resulting pH, depends on the strengths of the parent acid and base from which the salt is formed.

Here's how we classify salts based on their parent acid and base:

Salt of Strong Acid and Strong Base: Neither ion hydrolyzes significantly. The solution remains neutral, with pH $\approx 7$ .
Salt of Strong Acid and Weak Base: The cation (conjugate acid of the weak base) hydrolyzes to produce H $_3$ O $^+$ . The solution becomes acidic, with pH $< 7$ .
Salt of Weak Acid and Strong Base: The anion (conjugate base of the weak acid) hydrolyzes to produce OH $^-$ . The solution becomes basic, with pH $> 7$ .
Salt of Weak Acid and Weak Base: Both the cation and anion hydrolyze. The pH of the solution depends on the relative strengths of the conjugate acid and conjugate base, specifically on the comparison between the $K_a$ $K_{a}$ of the conjugate acid and the $K_b$ $K_{b}$ of the conjugate base.
- If $K_a(\text{conjugate acid}) > K_b(\text{conjugate base})$ , the solution is acidic.
- If $K_a(\text{conjugate acid}) < K_b(\text{conjugate base})$ , the solution is basic.
- If $K_a(\text{conjugate acid}) \approx K_b(\text{conjugate base})$ , the solution is nearly neutral.

Let's analyze each given salt:

KNO $_3$ (aq)
- Parent Acid and Base: KNO $_3$ is formed from a strong acid, nitric acid (HNO $_3$ ), and a strong base, potassium hydroxide (KOH).
- Hydrolysis: The K $^+$ ion is the conjugate acid of a strong base (KOH), and the NO $_3^-$ ion is the conjugate base of a strong acid (HNO $_3$ ). Neither K $^+$ nor NO $_3^-$ hydrolyzes significantly in water.
- Conclusion: The solution remains neutral.
  - pH $\approx 7$ .
CH $_3$ COONa (aq)
- Parent Acid and Base: CH $_3$ COONa is formed from a weak acid, acetic acid (CH $_3$ COOH), and a strong base, sodium hydroxide (NaOH).
- Hydrolysis: The Na $^+$ ion (from NaOH) does not hydrolyze. The CH $_3$ COO $^-$ ion (conjugate base of CH $_3$ COOH) hydrolyzes:

\text{CH}_3\text{COO}^- (aq) + \text{H}_2\text{O} (l) \rightleftharpoons \text{CH}_3\text{COOH} (aq) + \text{OH}^- (aq)

    This reaction produces hydroxide ions (OH$^-$).
*   **Conclusion**: The solution is basic.
    *   pH $> 7$.

3. NH $_4$ Cl (aq)

* Parent Acid and Base: NH $_4$ Cl is formed from a strong acid, hydrochloric acid (HCl), and a weak base, ammonia (NH $_3$ ).

* Hydrolysis: The Cl $^-$ ion (from HCl) does not hydrolyze. The NH $_4^+$ ion (conjugate acid of NH $_3$ ) hydrolyzes:

\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 (H$_3$O$^+$).
*   **Conclusion**: The solution is acidic.
    *   pH $< 7$.

4. C $_6$ H $_5$ COONH $_4$ (aq)

* Parent Acid and Base: C $_6$ H $_5$ COONH $_4$ is formed from a weak acid, benzoic acid (C $_6$ H $_5$ COOH), and a weak base, ammonia (NH $_3$ ).

* Hydrolysis: Both ions hydrolyze.

* The C $_6$ H $_5$ COO $^-$ ion (conjugate base of C $_6$ H $_5$ COOH) hydrolyzes to produce OH $^-$ :

\text{C}_6\text{H}_5\text{COO}^- (aq) + \text{H}_2\text{O} (l) \rightleftharpoons \text{C}_6\text{H}_5\text{COOH} (aq) + \text{OH}^- (aq)

    *   The NH$_4^+$ ion (conjugate acid of NH$_3$) hydrolyzes to produce H$_3$O$^+$: …

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Q.Arrange the following in increasing order of pH. KNO3 (aq), CH3COONa (aq), NH4Cl (aq), C6H5COONH4 (aq)

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Q.Arrange the following in increasing order of pH.
KNO3 (aq), CH3COONa (aq), NH4Cl (aq), C6H5COONH4 (aq)

Unlock everything free for 14 days