PYQ + Trends
1. TITLE
NEET 2025 Q1: Polyprotic Acids: The Ka Trend ($K_{a1} > K_{a2} > K_{a3}$) — Full solution + Why they asked this
2. THE ACTUAL NEET QUESTION
NEET 2025 — Question
Phosphoric acid ionizes in three steps with their ionization constant values $K_{a_{1}}$, $K_{a_{2}}$, and $K_{a_{3}}$, respectively, while K is the overall ionization constant. Which of the following statements are true?
A. $\log K = \log K_{a_{1}} + \log K_{a_{2}} + \log K_{a_{3}}$
B. $H_{3}PO_{4}$ is a stronger acid than $H_{2}PO_{4}^{-}$ and $HPO_{4}^{2-}$
C. $K_{a_{1}} > K_{a_{2}} > K_{a_{3}}$
D. $K_{a_{1}} = \frac{K_{a_{3}} + K_{a_{2}}}{2}$
Choose the correct answer from the options given below:
A) B, C and D only
B) A, B and C only
C) A and B only
D) A and C only
3. OFFICIAL ANSWER
NEET Answer Key: (B)
4. COMPLETE SOLUTION
Concept explanation & Step-by-step breakdown:
* Statement A (True): When a polyprotic acid like phosphoric acid ($H_3PO_4$) ionizes in successive steps, the overall reaction is the sum of the individual steps. The equilibrium constant for a net reaction obtained after adding multiple reactions equals the product of the individual constants. Therefore, $K = K_{a_{1}} \times K_{a_{2}} \times K_{a_{3}}$. Taking the logarithm on both sides yields the mathematical identity: $\log K = \log K_{a_{1}} + \log K_{a_{2}} + \log K_{a_{3}}$.
* Statement B (True): Successive proton removals get progressively harder, meaning the first intact acid molecule ($H_3PO_4$) is much more willing to donate a proton than its subsequent negatively charged ions ($H_{2}PO_{4}^{-}$ and $HPO_{4}^{2-}$). Thus, $H_3PO_4$ is the strongest acid among them.
* Statement C (True): The successive ionization constants become progressively smaller ($K_{a_{1}} > K_{a_{2}} > K_{a_{3}}$). This happens because it is far more difficult to remove a positively charged proton ($H^+$) from a negatively charged ion due to increasing electrostatic forces.
* Statement D (False): There is no arithmetic mean relationship between successive equilibrium constants. This is a fabricated distractor option.
Since statements A, B, and C are correct, Option B is the right answer.
5. WHY NEET ASKED THIS
Explain the examiner's intent:
* What concept were they testing? The examiner wanted to test two integrated concepts: the properties of polyprotic/polybasic acids (specifically the electrostatic reasoning behind $K_{a1} > K_{a2}$) and the mathematical laws of combining equilibrium constants.
* What's the common mistake they expected students to make? Students often forget their logarithm properties and might assume that adding reactions means adding the $K$ values (instead of multiplying them), leading them to incorrectly mark Statement A as false.
* Which option was the "trap"? Option A (B, C and D only) is the trap for students who correctly identify the polyprotic trend but fail to recognize the logarithmic identity in Statement A.
6. COMPARISION TABLE
PATTERN ALERT: Questions combining multiple equilibrium constants ($K_1, K_2, K_3$) repeat frequently.
| Year | How they asked | Twist |
| :--- | :--- | :--- |
| 2025 | Identify true statements about Phosphoric acid steps | Integrated logarithms with the chemical property of polyprotic acids. |
| 2024 (Re) | Find the relation between $K_1$ and $K_2$ for given equilibria | Required reversing and multiplying a reaction to find $K_2 = \frac{1}{\sqrt{K_1}}$. |
| 2017 | Calculate overall $K$ for oxidation of $NH_3$ from 3 separate steps | Required adding three equations and multiplying their constants ($K_{NET} = \frac{K_2 K_3^3}{K_1}$). |
Prediction for NEET 2026:
Expect a numerical calculation where you are given the exact values of $K_{a1}$ and $K_{a2}$ for a dibasic acid (like $H_2S$ or $H_2CO_3$) and asked to calculate the concentration of the final anion (e.g., $S^{2-}$) in the solution, requiring the use of the overall $K = K_{a1} \times K_{a2}$.
7. SIMILAR QUESTIONS TO PRACTICE
"If you understood this, you can solve:"
* NEET 2024 Re Q5: Relation between equilibrium constants when stoichiometry is halved and reversed.
* NEET 2017 Q31: Finding the overall equilibrium constant from three sequential gaseous reactions.
* NCERT Based Variation: Calculating the second dissociation constant of $H_2S$ using common ion effect.
8. QUICK REVISION FOR THIS TYPE
For Polyprotic Acid & Equilibrium Constant questions, always:
* First check how many steps the reaction is broken into. To find the overall $K$, you must multiply the individual step constants ($K_{overall} = K_1 \times K_2 \times \dots$).
* Then apply log rules if requested: $\log(K_{overall}) = \log K_1 + \log K_2 + \dots$
* Watch out for the universal trend in polybasic acids: $K_{a1} > K_{a2} > K_{a3}$. Removing a positive proton from an increasingly negative ion requires overcoming strong electrostatic forces, making each step less favorable.
9. NCERT BASE
This question tests: NCERT Class 11, Chemistry, Chapter: Equilibrium, Polybasic Acids and Polyacidic Bases section (Reference: Page 199-200).
10. CTA
All PYQs from this chapter → Equilibrium Guide
Practice similar questions → [PYQ Test: Equilibrium]