Conjugate acid of a weak base is always stronger. What will be the decreasing order of basic strength of the f

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Q. Conjugate acid of a weak base is always stronger. What will be the decreasing order of basic strength of the following conjugate bases? OH^-, RO^-, CH3COO^-, Cl^-

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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 key idea is that the stronger the acid, the weaker its conjugate base. By comparing the acid strengths of the parent acids (H₂O, ROH, CH₃COOH, HCl), we can rank the conjugate bases in decreasing order of basic strength: RO⁻ > OH⁻ > CH₃COO⁻ > Cl⁻.

Concept and Intuition

The Brønsted-Lowry theory defines an acid as a proton donor and a base as a proton acceptor. When an acid donates a proton, it forms its conjugate base. The strength of an acid is inversely related to the strength of its conjugate base: a strong acid has a weak conjugate base, and a weak acid has a strong conjugate base.

Here, we are given four conjugate bases: OH⁻, RO⁻, CH₃COO⁻, and Cl⁻. To rank their basic strength, we need to consider the strength of their parent acids. The parent acids are:

OH⁻ comes from H₂O (water)
RO⁻ comes from ROH (alcohol)
CH₃COO⁻ comes from CH₃COOH (acetic acid)
Cl⁻ comes from HCl (hydrochloric acid)

The stronger the parent acid, the weaker the conjugate base. So, we need to compare the acid strengths of H₂O, ROH, CH₃COOH, and HCl.

Step-by-Step Reasoning

Identify the parent acids and their relative strengths.

The four parent acids are:
- HCl: a strong acid (completely dissociates in water)
- CH₃COOH: a weak acid (partial dissociation, Ka ≈ 1.8 × 10⁻⁵)
- H₂O: a very weak acid (Ka ≈ 1.0 × 10⁻¹⁴)
- ROH (alcohol): an even weaker acid than water (Ka ≈ 10⁻¹⁶ to 10⁻¹⁸)
So, the order of acid strength is:

\text{HCl} > \text{CH}_3\text{COOH} > \text{H}_2\text{O} > \text{ROH}

Apply the inverse relationship between acid strength and conjugate base strength. Since a stronger acid gives a weaker conjugate base, the order of basic strength of the conjugate bases will be the reverse of the acid strength order. Therefore:
- The weakest acid (ROH) gives the strongest conjugate base (RO⁻).
- The next weakest acid (H₂O) gives the next strongest conjugate base (OH⁻). …

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Q.Conjugate acid of a weak base is always stronger. What will be the decreasing order of basic strength of the following conjugate bases? OH^-, RO^-, CH3COO^-, Cl^-

Concept and Intuition

Step-by-Step Reasoning

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Q.Conjugate acid of a weak base is always stronger. What will be the decreasing order of basic strength of the following conjugate bases?
OH^-, RO^-, CH3COO^-, Cl^-

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