What will be the conjugate bases for the Brönsted acids: HF, H2SO4 and HCO– 3?

Exercises · 6.37

Q. What will be the conjugate bases for the Brönsted acids: HF, H2SO4 and HCO- 3?

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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 conjugate base of a Brønsted acid is what remains after the acid donates a proton ( $\text{H}^+$ ). For HF, it is $\text{F}^-$ ; for $\text{H}_2\text{SO}_4$ , it is $\text{HSO}_4^-$ ; and for $\text{HCO}_3^-$ , it is $\text{CO}_3^{2-}$ .

The Brønsted-Lowry theory defines an acid as a proton donor. When an acid gives away a proton ( $\text{H}^+$ ), it transforms into its conjugate base — the species that can potentially accept a proton back. The relationship is simple: conjugate base = acid minus one $\text{H}^+$ .

The trick is to subtract exactly one proton, and to adjust the charge correctly. A proton has a charge of $+1$ , so removing it reduces the positive charge (or increases the negative charge) by one unit.

Let’s apply this to each acid.

HF (Hydrofluoric acid)

Remove one $\text{H}^+$ from $\text{HF}$ .

$\text{HF} - \text{H}^+ = \text{F}^-$

The fluorine atom retains the electron pair that was shared with the proton, giving it a negative charge.

Conjugate base: $\text{F}^-$ (fluoride ion).
$\text{H}_2\text{SO}_4$ (Sulfuric acid)

This is a diprotic acid — it can donate two protons, but we only remove one to get the conjugate base.

$\text{H}_2\text{SO}_4 - \text{H}^+ = \text{HSO}_4^-$

The hydrogen sulfate ion ( $\text{HSO}_4^-$ ) is the conjugate base of sulfuric acid.

Watch out

A common mistake is to remove both protons at once, giving $\text{SO}_4^{2-}$ . That is the conjugate base of $\text{HSO}_4^-$ , not of $\text{H}_2\text{SO}_4$ . Always remove exactly one $\text{H}^+$ for the first conjugate base. …

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Q.What will be the conjugate bases for the Brönsted acids: HF, H2SO4 and HCO– 3?

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