Q.What will be the conjugate bases for the following Brönsted acids: HF, H 2SO4 and HCO3 – ?
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 (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
Acid⇌Conjugate Base+H+
Base+H+⇌Conjugate Acid
A conjugate acid-base pair consists of two species that differ by exactly one proton (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.
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:
HCl+H2O→Cl−+H3O+
Let's identify the pairs:
- Pair 1: HCl (acid) and Cl− (conjugate base). They differ by one H+.
- Pair 2: H2O (base) and H3O+ (conjugate acid). They also differ by one 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.
To find the conjugate base of any acid, simply remove one H+ and reduce the charge by +1.
To find the conjugate acid of any base, add one H+ and increase the charge by +1.
A Quick Reference Table
| Acid | Conjugate Base | Base | Conjugate Acid |
|---|---|---|---|
| HCl | Cl− | NH3 | NH4+ |
| H2SO4 | HSO4− | H2O | H3O+ |
| NH4+ | NH3 | OH− | H2O |
| H2O | OH− | CO32− | HCO3− |
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
The concept of conjugate pairs explains why some acids are "strong" and others "weak". A strong acid (like HCl) has a very weak conjugate base (Cl−) — it has almost no tendency to grab back the proton. A weak acid (like acetic acid, CH3COOH) has a stronger conjugate base (CH3COO−) — it wants the proton back more.
This relationship is inverse: the stronger the acid, the weaker its conjugate base, and vice versa.
A common mistake is to think that the conjugate base of a strong acid is itself a strong base. It is not — it is extremely weak. Cl− is not a base you'd ever notice in water.
The Big Picture
Every time you see an acid, ask: "What does it become after losing its proton?" That's its conjugate base. Every time you see a base, ask: "What does it become after gaining a proton?" That's its conjugate acid. These two are always a pair, always linked, and always present in any acid-base reaction.
The Brønsted–Lowry theory turns acid-base chemistry into a simple exchange: proton donor + proton acceptor → conjugate base + conjugate acid. Learn to spot the pairs, and you've unlocked the entire framework.
This topic is commonly searched as "Brønsted Lowry Conjugate Pairs 11 chemistry important questions" or "Brønsted Lowry Conjugate Pairs formula and examples", and it maps cleanly onto the Class 11 Chemistry portion of the NCERT/CBSE syllabus. Because brønsted lowry conjugate pairs shows up repeatedly in JEE Main, NEET and state CET Chemistry papers, mastering the underlying idea (not just the formula) is genuinely worth the extra time.
The key idea is that a conjugate base is formed when a Brønsted-Lowry acid donates a proton (H+). The conjugate base is simply the acid molecule minus one proton.
Step 1: For HF, remove one H+.
Step 2: For H2SO4, remove one H+ — note that sulfuric acid is diprotic, so its first conjugate base is the bisulfate ion.
Step 3: For HCO3−, remove one H+ — this gives the carbonate ion.
The conjugate bases are F−, HSO4−, and CO32− respectively.
The conjugate base of a Brønsted acid is what remains after the acid donates a proton (H+). For HF, it is F−; for H2SO4, it is HSO4−; for HCO3−, it is CO32−.
The idea is simple: a Brønsted acid is a proton donor. When it gives away that H+, the species left behind is its conjugate base. The charge changes by exactly +1 (since the proton has a +1 charge). So, to find the conjugate base, just remove one H+ and adjust the charge accordingly.
Let’s apply this to each acid.
-
HF (Hydrofluoric acid)
Remove one H+ from HF. You are left with F.
The original molecule is neutral; losing a +1 charge leaves a −1 charge.
So the conjugate base is F− (fluoride ion).
-
H2SO4 (Sulfuric acid)
This is a diprotic acid — it can donate two protons, but here we only consider the first donation.
Remove one H+ from H2SO4. What remains is HSO4.
The original molecule is neutral; after losing a +1 charge, the charge becomes −1.
So the conjugate base is HSO4− (hydrogen sulfate ion, also called bisulfate).
Watch outA common mistake is to remove both protons at once and write SO42− as the conjugate base of H2SO4. That is incorrect for this question — SO42− is the conjugate base of HSO4−, not of H2SO4 itself. Always remove exactly one proton.
-
HCO3− (Bicarbonate ion)
This one already carries a negative charge. Remove one H+ from it.
You are left with CO3.
The original charge is −1; losing a +1 charge makes the new charge −2.
So the conjugate base is CO32− (carbonate ion).
A quick check: the sum of charges on the conjugate base and the proton (+1) must equal the charge on the original acid. For HCO3− (charge −1), conjugate base charge +(+1)=−1, so conjugate base charge must be −2. Works every time.
The conjugate bases are F−, HSO4−, and CO32−, respectively.
Showing the 12 most recent of 26 on this concept.
- CBSE 2026Set ANNUAL1 markMCQQ.Which one is the conjugate base of HSO4-?(a) H2SO4(b) HSO3(c) SO3(d) SO4^2-
›Reveal solutionSolution
A conjugate base is formed by removing one H+ (proton) from an acid. HSO4- losing a proton gives SO4^2-.
By the Bronsted-Lowry definition, an acid's conjugate base is what remains after the acid donates (loses) a proton (H+).
HSO4- (hydrogen sulfate ion) donating a proton:
HSO4- -> H+ + SO4^2-
The species left behind, SO4^2- (sulfate ion), is the conjugate base of HSO4-.
(Note: H2SO4 is actually the conjugate ACID of HSO4-, not its conjugate base -- H2SO4 is what you'd get if HSO4- instead GAINED a proton.)
✓Final answer(d) SO4^2-.
- CBSE 2026Set ANNUAL1 markQ.Write the conjugate base of HClO4.
›Reveal solutionSolution
Removing one H+ from HClO4 gives its conjugate base, ClO4- (perchlorate ion).
By the Bronsted-Lowry definition, an acid is a proton (H+) donor, and after it donates a proton, the resulting species (which can, in principle, accept a proton back) is called its conjugate base. HClO4 (perchloric acid) donates one H+: HClO4 → H+ + ClO4-. The species remaining, ClO4- (perchlorate ion), is the conjugate base of HClO4. (HClO4 is one of the strongest common acids, so ClO4- is correspondingly an extremely weak conjugate base.)
✓Final answerThe conjugate base of HClO4 is ClO4- (perchlorate ion).
- CBSE 2026Set ANNUAL1 markMCQQ.Which of the following is a Lewis base ?(a) HCl(b) HNO3(c) HF(d) NH3
›Reveal solutionSolution
NH3 is the Lewis base (electron-pair donor).
A Lewis base is an electron-pair donor. NH3 has a lone pair of electrons on nitrogen which it can donate to an electron-deficient species, so it is a Lewis base. HCl, HNO3 and HF donate protons — they are (Bronsted) acids, and as electron-pair acceptors behave as Lewis acids.
✓Final answer(D) NH3.
- CBSE 2026Set ANNUAL1 markMCQQ.The conjugate base of HBr is(a) H2Br^+(b) H^+(c) Br^-(d) Br^+
›Reveal solutionSolution
Conjugate base of HBr = Br-.
According to the Bronsted-Lowry concept, the conjugate base is what remains after an acid donates one proton. HBr → H+ + Br-, so the conjugate base of HBr is the bromide ion, Br-.
✓Final answer(C) Br-.
- CBSE 2026Set ANNUAL1 markMCQQ.Which of the following is the strongest base ?(a) AsH3(b) NH3(c) PH3(d) SbH3
›Reveal solutionSolution
NH3 is the strongest base among NH3, PH3, AsH3 and SbH3.
Basicity of these hydrides depends on the availability of the lone pair on the central atom. As we go down group 15 (N → P → As → Sb) the atom gets larger, the lone pair spreads over a larger volume and becomes less available for donation. So basic strength decreases: NH3 > PH3 > AsH3 > SbH3. Hence NH3 is the strongest base.
✓Final answer(B) NH3.
- CBSE 2025Set ANNUAL1 markMCQQ.Which of the following bases is weakest?(a) C2H5O-(b) NO3-(c) I-(d) CH3COO-
›Reveal solutionSolution
I- is the weakest base of the four options, being the conjugate base of the very strong acid HI.
The strength of a conjugate base is inversely related to the strength of its parent (conjugate) acid — the stronger the acid, the weaker its conjugate base.
- C2H5O- is the conjugate base of ethanol (a very weak acid), so C2H5O- is a fairly strong base.
- CH3COO- is the conjugate base of acetic acid (a weak acid), so it is a moderately weak base.
- NO3- is the conjugate base of HNO3 (a strong acid), so it is a very weak base.
- I- is the conjugate base of HI, which is an even stronger acid than HNO3 (HI is one of the strongest hydrohalic acids), making I- an even weaker base than NO3-.
So I- is the weakest base among the four.
✓Final answer(C) I-.
- CBSE 2025Set ANNUAL1 markMCQQ.Which one of the following is not an amphoteric substance?(a) HNO3(b) HCO3-(c) H2O(d) NH3
›Reveal solutionSolution
HNO3 is the only substance among the options that is not amphoteric.
An amphoteric substance can donate a proton (act as an acid) as well as accept a proton (act as a base).
-
HCO3- can donate a proton to become CO3^2- (acting as an acid) or accept a proton to become H2CO3 (acting as a base) — amphoteric.
-
H2O can donate a proton to become OH- (acid) or accept a proton to become H3O+ (base) — amphoteric.
-
NH3 can accept a proton to become NH4+ (base) and, in the presence of a much stronger base, can also donate a proton to form NH2- (acid) — amphoteric.
-
HNO3 is a strong acid; it readily donates its proton but has essentially no tendency to accept a proton (it is not itself a base under ordinary conditions), so it is not amphoteric.
✓Final answer(A) HNO3.
-
- CBSE 2025Set ANNUAL1 markMCQQ.The conjugate acid of NH3 is(a) NH2^-(b) NH4^+(c) NH^2-(d) N2H4
›Reveal solutionSolution
A conjugate acid is simply the base plus one extra proton (H+); adding H+ to NH3 gives the ammonium ion, NH4+.
According to the Bronsted-Lowry theory of acids and bases: an acid is a proton (H+) donor, and a base is a proton acceptor. When a base accepts a proton, it becomes its 'conjugate acid' — a species that could, in the reverse reaction, donate that proton back.
NH3 acts as a base here (it has a lone pair on nitrogen that can accept a proton):
NH3 + H+ -> NH4+
So NH4+ (the ammonium ion) is the conjugate acid of NH3, formed by adding exactly one proton to the nitrogen's lone pair.
The other options don't fit: NH2- would be formed by REMOVING a proton from NH3 (making NH3 the conjugate acid of NH2-, i.e. NH2- is NH3's conjugate BASE, the opposite relationship); NH^2- and N2H4 don't correspond to a simple single-proton addition to NH3.
✓Final answer(b) NH4^+.
- CBSE 2025Set ANNUAL1 markQ.Fill in the blank: The conjugate base of NH2- is ___________.
›Reveal solutionSolution
Removing one proton from the amide ion NH2- gives NH^2- (imide ion), which is therefore its conjugate base. Removing a further H+ would give N^3-, which is the conjugate base of NH^2-, not of NH2-.
By the Bronsted-Lowry definition, a conjugate base is formed by removing exactly one proton (H+) from a species.
Starting from the amide ion NH2- (which has two H and a charge of -1), removing one H+ leaves NH^2- (the imide / azanide ion, charge -2):
NH2- - H+ -> NH^2-
The full stepwise deprotonation ladder is NH3 -> NH2- -> NH^2- -> N^3-, each step removing one proton. So N^3- (nitride ion) is the conjugate base of NH^2-, NOT of NH2-. The stored answer N^3- incorrectly removed two protons.
✓Final answerNH^2- (imide / azanide ion).
- CBSE 2025Set ANNUAL1 markMCQQ.Assertion (A): The conjugate base of a strong acid is a weak base. Reason (R): Strong acids completely dissociate in water, leaving their conjugate bases with low tendency to accept protons.(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).(b) Both (A) and (R) are true, but (R) is not the correct explanation of (A).(c) (A) is true, but (R) is false.(d) (A) is false, but (R) is true.
›Reveal solutionSolution
Strong acids ionize almost completely in water. The species left behind after losing a proton (the conjugate base) has little affinity for that proton, i.e., a low tendency to accept a proton back — which is exactly the definition of a weak base. So (R) correctly explains (A).
Assertion (A): The conjugate base of a strong acid is a weak base — this is correct. For example, HCl (strong acid) dissociates completely to give Cl-, which shows essentially no tendency to accept a proton back in water (Cl- is a very weak base).
Reason (R): Strong acids completely dissociate in water, leaving their conjugate bases with a low tendency to accept protons — this is also correct and is precisely the mechanism behind (A).
The conjugate acid-base pair relationship states that the stronger the acid, the weaker its conjugate base (and vice versa). Since a strong acid has an overwhelming tendency to donate its proton (complete dissociation, large Ka), the reverse process — its conjugate base accepting a proton back — is correspondingly weak. This is exactly what (R) states, so (R) is the correct explanation of (A).
✓Final answerThe correct option is (a) — both (A) and (R) are true, and (R) is the correct explanation of (A).
- CBSE 2025Set hz1 markMCQQ.Select the correct one: Which of the following is a strongest conjugate base?(a) Cl-(b) SO4^2-(c) CH3COO-(d) NO3-
›Reveal solutionSolution
Acid strength and conjugate base strength are inversely related — the weaker the parent acid, the stronger its conjugate base. CH3COO- comes from the weakest acid here (CH3COOH), so it is the strongest conjugate base.
Each ion's parent acid:
- Cl- comes from HCl, a strong acid → Cl- is an extremely weak (negligible) base.
- SO4^2- comes from HSO4- (itself a fairly strong acid) → SO4^2- is a weak base.
- CH3COO- comes from CH3COOH (acetic acid), a weak acid (Ka ~ 1.8x10^-5) → CH3COO- is a comparatively strong (moderate) base.
- NO3- comes from HNO3, a strong acid → NO3- is a negligible/very weak base.
Since CH3COOH is by far the weakest acid among HCl, HSO4-, CH3COOH and HNO3, its conjugate base CH3COO- holds protons the most strongly among these four, making it the strongest conjugate base.
✓Final answerThe strongest conjugate base is (c) CH3COO-.
- CBSE 2025Set ANNUAL1 markMCQQ.Water is(a) protophobic substance(b) protophilic substance(c) amphoteric substance(d) aprotic substance
›Reveal solutionSolution
Water can both donate and accept a proton, so it is an amphoteric (amphiprotic) substance.
In the Brønsted–Lowry sense:
- Water acting as an acid (proton donor): H2O+NH3→OH−+NH4+
- Water acting as a base (proton acceptor): H2O+HCl→H3O++Cl−
A species that can behave as both an acid and a base is called amphoteric (or amphiprotic). "Protophilic" describes a species with a strong tendency to accept protons only, and "protophobic"/"aprotic" describe solvents that resist donating or accepting protons at all — none of these fully describe water's dual behaviour the way "amphoteric" does.
✓Final answer(c) amphoteric substance
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