Grade 11 Physical Sciences · Chemical Change

Identify conjugate acid-base pairs

Find the acid, the base and their conjugates in a proton-transfer reaction (in NH₃ + H₂O, NH₄⁺ is the conjugate acid).

past paper questions use it: 15 past paper questions use it
videos: 3 videos

Learn it: identify conjugate acid-base pairs

Pick one and watch it, then try the questions below. Videos load only when you tap play.

Conjugate acid base pairs: Grade 11 and 12 Acids and BasesMiss Martins Maths and Science

Conjugate acid base pairs PRACTICE: Grade 11 and 12 Acids and BasesMiss Martins Maths and Science

Identify Acid and Base Grade 11Kevinmathscience

Now try it on a real exam question

No sign-up. Takes 3 minutes.

Question 1.7 · May/June 2025 · Paper 2 · National (DBE) · 2 marks

1.7 Consider the equation for the acid-base reaction below.

HSO_{4}^{-} (aq) + NH_{3} (aq) ⇌ SO_{4}^{2 -} (aq) + NH_{4}^{+} (aq)

The two acids in this reaction are ...(2)

A $NH_{3}$ (aq) and $NH_{4}^{+}$ (aq)
B $HSO_{4}^{-}$ (aq) and $SO_{4}^{2 -}$ (aq)
C $HSO_{4}^{-}$ (aq) and $NH_{3}$ (aq)
D $HSO_{4}^{-}$ (aq) and $NH_{4}^{+}$ (aq)

Work it out on paper first. Then mark yourself with the official memo, one tick at a time.

More past paper questions: identify conjugate acid-base pairs

Work each one out first, then open the memo and check every tick.

3. Question 1.8 · May/June 2022 · Paper 2 · National (DBE) · 2 marks
1.8 Consider the ionisation reactions I and II.

I: $H_{2} PO_{4}^{-} + H_{2} O (ℓ) ⇌ H_{3} O^{+} (aq) + X$

II: $X + H_{2} O (ℓ) ⇌ H_{3} O^{+} (aq) + Y$

Which ONE of the following combinations represents the formulae of X and Y respectively?(2)
1. A X: $HPO_{4}^{2 -}$ ; Y: $PO_{4}^{3 -}$
2. B X: $HPO_{4}^{2 -}$ ; Y: $H_{3} PO_{4}$
3. C X: $H_{3} PO_{4}$ ; Y: $PO_{4}^{3 -}$
4. D X: $HPO_{4}^{2 -}$ ; Y: $H_{2} PO_{4}^{-}$
Show the memo (1 tick)Memo
1. ✓
  Remove one H+ to get the conjugate baseIn each reaction the acid donates one $H^{+}$ to water, so its conjugate base has one H fewer and a charge one more negative.
  I: $H_{2} PO_{4}^{-} \to HPO_{4}^{2 -}$ , so X $= HPO_{4}^{2 -}$ .
  II: $HPO_{4}^{2 -} \to PO_{4}^{3 -}$ , so Y $= PO_{4}^{3 -}$ .
  
  Answer: A
  2 marks · Identify conjugate acid-base pairs
4. Question 7.1.3 · November 2022 · Paper 2 · National (DBE) · 2 marks
7.1 Ethanoic acid is a weak acid that reacts with water according to the following balanced equation:

$CH_{3} COOH(aq) + H_{2} O (ℓ) ⇌ CH_{3} COO^{-} (aq) + H_{3} O^{+} (aq)$
7.1.3 Write down the formulae of the TWO bases in the equation above.(2)
Show the memo (2 ticks)Memo
1. ✓
  First base $H_{2} O$
  1 mark · Identify conjugate acid-base pairs
2. ✓
  Second base $CH_{3} COO^{-}$
  1 mark · Identify conjugate acid-base pairs
5. Question 1.8 · November 2025 · Paper 2 · Provincial · 2 marks
Various options are provided as possible answers to the following questions. Choose the answer and write only the letter (A–D) next to the question numbers (1.1 to 1.10) in the ANSWER BOOK, for example 1.11 E.
1.8 Consider the following equation:

$(COOH)_{2}$ (aq) + $H_{2} O$ ( $ℓ$ ) ⇌ $(COO)_{2}^{2 -}$ (aq) + $H_{3} O^{+}$ (aq)

The bases in the above reaction are ...(2)
1. A $(COOH)_{2}$ and $H_{2} O$
2. B $(COO)_{2}^{2 -}$ and $H_{3} O^{+}$
3. C $H_{2} O$ and $(COO)_{2}^{2 -}$
4. D $(COOH)_{2}$ and $(COO)_{2}^{2 -}$
Show the memo (1 tick)Memo
1. ✓
  Choose the correct optionA base is a proton ( $H^{+}$ ) acceptor. In the forward reaction $H_{2} O$ accepts protons to form $H_{3} O^{+}$ . In the reverse reaction $(COO)_{2}^{2 -}$ accepts protons to form $(COOH)_{2}$ . So the bases are $H_{2} O$ and $(COO)_{2}^{2 -}$ .
  
  Answer: C
  2 marks · Identify conjugate acid-base pairs
6. Question 8.2.1 · November 2025 · Paper 2 · Provincial · 2 marks
Consider the reactions below:

$HSO_{4}^{-} (aq) + H_{2} O (ℓ) ⇌ H_{3} O^{+} (aq) + SO_{4}^{2 -} (aq)$
8.2 From the reaction above, write down the:
8.2.1 Formulae of the TWO substances that can act as ampholytes(2)
Show the memo (2 ticks)Memo
1. ✓
  First ampholyte $HSO_{4}^{-}$
  1 mark · Identify conjugate acid-base pairs
2. ✓
  Second ampholyte $H_{2} O$
  1 mark · Identify conjugate acid-base pairs

Free on Android

Know exactly which skill is costing you marks.

Mark past papers tick by tick in the NavyBlue app. It remembers every mark you lose, shows you the skills to fix first, and picks your next questions.

The NavyBlue app showing a learner's weak skills