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1. TITLE

NEET 2019 Q24: Conjugate Pairs & Strength — Full solution + Why they asked this

2. THE ACTUAL NEET QUESTION

NEET 2019 — Question
Conjugate base for Bronsted acids $H_2O$ and $HF$ are:
A) $H_3O^+$ and $H_2F^+$, respectively
B) $OH^-$ and $H_2F^+$, respectively
C) $H_3O^+$ and $F^-$, respectively
D) $OH^-$ and $F^-$, respectively.

3. OFFICIAL ANSWER

NEET Answer Key: (D)

4. COMPLETE SOLUTION

Concept Explanation:
According to the Brönsted-Lowry concept, an acid is a proton ($H^+$) donor and a base is a proton acceptor. When an acid donates a single proton, the remaining species is called its conjugate base.

To find the conjugate base of any species, simply remove one $H^+$ ion:
* For $H_2O$: Removing an $H^+$ leaves the hydroxyl ion, $OH^-$.
* For $HF$: Removing an $H^+$ leaves the fluoride ion, $F^-$.

Why other options are wrong:
Options A, B, and C contain $H_3O^+$ and $H_2F^+$. These are formed by adding a proton to $H_2O$ and $HF$, making them the conjugate acids, not the conjugate bases.

5. WHY NEET ASKED THIS

Explain the examiner's intent:
* What concept were they testing? The core fundamentals of the Brönsted-Lowry acid-base theory and the ability to identify conjugate pairs.
* What's the common mistake they expected students to make? Students often confuse conjugate acids with conjugate bases in the heat of the exam, blindly adding an $H^+$ instead of subtracting it.
* Which option was the "trap"? Option A was the primary trap, as it lists the perfectly correct conjugate acids ($H_3O^+$ and $H_2F^+$) for students who misread the question.

6. COMPARISION TABLE

Year How they asked Twist
2019 Identify conjugate bases for $H_2O$ and $HF$. Paired conjugate acids in the options to test attention to detail.
2000 Find the conjugate acid of $NH_2^-$. Required adding a proton to an already negatively charged ion (Answer: $NH_3$).
1999 Identify the strongest conjugate base from a list of ions. Required knowing the inverse strength rule: the weakest acid produces the strongest conjugate base.

Prediction for NEET 2026:
Expect a variation that blends conjugate identification with the mathematical relationship of their strengths. You might be given the $K_a$ or $pK_a$ of an acid and asked to calculate the $pK_b$ of its conjugate base using the formula $pK_a + pK_b = 14$.

7. SIMILAR QUESTIONS TO PRACTICE

"If you understood this, you can solve:"
* NEET 2000 Q102: Conjugate acid of $NH_2^-$
* NEET 1999 Q105: The strongest conjugate base among $SO_4^{2-}$, $Cl^-$, $NO_3^-$, and $CH_3COO^-$
* Topic with variation: Calculating the $pK_b$ of a conjugate base like $CH_3COO^-$ when given the $pK_a$ of acetic acid ($4.75$).

8. QUICK REVISION FOR THIS TYPE

For Conjugate Pair Strength questions, always:

  • [First check] Exactly what is being asked—conjugate acid or conjugate base?
  • [Then apply] The simple math:
    • Conjugate Base = Acid $-$ $H^+$
    • Conjugate Acid = Base $+$ $H^+$
  • [Watch out for] The Inverse Strength Rule! A strong acid always yields a very weak conjugate base, and a weak acid yields a strong conjugate base. If the question becomes numerical, remember that for any conjugate acid-base pair at $298K$: $K_a \times K_b = K_w$ or $pK_a + pK_b = 14$.

9. NCERT BASE

This question tests: NCERT Chemistry, Chapter 6-Equilibrium, Section 6.10.2 (The Brönsted-Lowry Acids and Bases).

10. CTA

All PYQs from this chapter → Equilibrium Guide
Practice similar questions → [PYQ Test: 6-Equilibrium]