📋 Given Parameters & Problem Setup
Step 1 Ionization Constant K_{a1}
Step 2 Ionization Constant K_{a2}
Step 3 Ionization Constant K_{a3}
Overall Constant K = K_{a1} · K_{a2} · K_{a3}
Statements A: log K = log K_{a1} + log K_{a2} + log K_{a3}; B: H₃PO₄ stronger than H₂PO₄⁻ and HPO₄²⁻; C: K_{a1} > K_{a2} > K_{a3}; D: K_{a1} = (K_{a3} + K_{a2})/2
Overall K is the product: K = K₁ K₂ K₃ ⟹ log K = Σ log K_i (A is TRUE)! Neutral acid loses H⁺ much easier than negative anions ⟹ H₃PO₄ is strongest (B is TRUE)! Stepwise constants drop by orders of magnitude: K_{a1} >> K_{a2} >> K_{a3} (C is TRUE)! Arithmetic mean D is absurd! ⟹ A, B, and C only!
1 Recall polyprotic acid equilibrium: For multi-step ionization, the overall equilibrium constant is the PRODUCT of stepwise constants: K = K_{a1} × K_{a2} × K_{a3}.
2 Taking log on both sides: log K = log K_{a1} + log K_{a2} + log K_{a3}. Statement A is TRUE.
3 Acid strength comparison: Removing a proton from neutral H₃PO₄ requires far less energy than from negatively charged H₂PO₄⁻ or doubly charged HPO₄²⁻. Thus H₃PO₄ is a stronger acid than its conjugate species. Statement B is TRUE.
4 Stepwise constants: K_{a1} (7.5 × 10⁻³) >> K_{a2} (6.2 × 10⁻⁸) >> K_{a3} (4.2 × 10⁻¹³). Statement C is TRUE.
5 Statement D proposes an arithmetic average between constants differing by 10⁵, which is mathematically false.
6 Conclusion: A, B and C only are true.
7 Match Option (4) in under 15 seconds!
🛡️ Why You Can 100% Trust This Shortcut: Successive Ionization of Polybasic Acids (NCERT Chemistry · Equilibrium: K_a1 >> K_a2 >> K_a3).
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Some students hesitate on Statement B, wondering if an amphiprotic anion could somehow act as a stronger acid. Electrostatic attraction ensures neutral parent acid is ALWAYS much stronger than its negative conjugate anions!
🛡️ How to avoid the -1 mark: Law of polyprotic acids: Neutral acid is always strongest: K_{a1} >> K_{a2} >> K_{a3}!
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NEET Chemistry Core Syllabus · Equilibrium Standard Benchmark
Core Scientific Principle Tested:
"K = K₁·K₂·K₃ ⟹ log K = Σ log K_i; K_{a1} > K_{a2} > K_{a3}."
Syllabus Focus: Ionization of Polybasic Acids and Overall Equilibrium Constant
K multiplies (A), neutral is strongest (B), K drops stepwise (C) ⟹ A, B, C are TRUE!
