pH Calculations — Strong acid/base, weak acid/base
Ionic Equilibrium
6
JEE Qs
8%
Hard
75
min
Master the art of identifying the primary source of H+ or OH- ions and judiciously applying approximations or the full quadratic solution based on the problem's context.
🧮 Key Formulas
✅ Key Points for JEE
- 1Strong acids and bases dissociate completely in aqueous solutions; their [H+] or [OH-] can be directly calculated from their molar concentration and stoichiometry.
- 2Weak acids and bases establish an equilibrium with their dissociated ions, governed by their respective dissociation constants (Ka or Kb). Use ICE tables or approximations.
- 3For very dilute strong acid/base solutions (concentration <= 10^-7 M), the autoionization of water must be considered alongside the acid/base contribution to calculate total [H+] or [OH-].
- 4Always check the validity of approximations for weak acid/base calculations (e.g., [HA] at equilibrium ≈ C_HA or percentage ionization < 5%) to avoid significant errors.
- 5The relationship pH + pOH = 14 and Ka * Kb = Kw are crucial for interconverting between acid and base properties and solving various equilibrium problems.
⚠️ Common Mistakes
- ✕Ignoring the autoionization of water for extremely dilute strong acid/base solutions, leading to incorrect pH values (e.g., pH > 7 for dilute acid).
- ✕Incorrectly applying the approximation ([HA] >> [H+]) for weak acids/bases when the concentration is not sufficiently large compared to Ka/Kb, or when percent ionization is high.
- ✕Mistakes in logarithmic calculations, especially with negative signs and handling powers of 10.
- ✕Confusing Ka with Kb, or [H+] with [OH-], leading to incorrect pH/pOH calculations.
📝 Practice Questions
See allQ74.If 1 mM solution of ethylamine produces pH = 9, then the ionization constant (Kb) of ethylamine is 10−x . The value of x is ______ (nearest integer). [The degree of ionization of ethylamine can be neglected with respect to unity.]
Q61. Ksp for Cr(OH)3 is 1.6 × 10−30 . What is the molar solubility of this salt in water? (1) 1.8×10−30 (2) 5√1.8 × 10−30 27 × 10−30 (3) 4√1.6×10−3027 (4) 2√1.6
Q58.The molar solubility(s) of zirconium phosphate with molecular formula (Zr4+)3(PO3−4 )4 is given by relation : (1) Ksp 13 (2) Ksp 17 ( 9612 ) ( 6912 ) (3) Ksp 17 (4) Ksp 16 ( 8435 ) ( 5348 )
Q62.Which of the following happens when NH4OH is added gradually to the solution containing 1 M A2+ and 1MB3+ ions? Given : Ksp [A(OH)2] = 9 × 10−10 and Ksp [B(OH)3] = 27 × 10−18 at 298 K . (1) Both A(OH)2 and B(OH)3 do not show (2) A(OH)2 will precipitate before B(OH)3 precipitation with NH4OH (3) B(OH)3 will precipitate before A(OH)2 (4) A(OH)2 and B(OH)3 will precipitate together excess HCHO alkali−
Q68.pH of water is 7 at 25∘C. If water is heated to 80∘C., it's pH will : (1) Decrease (2) H+ concentration increases, OH− concentration decreases (3) Remains the same (4) Increase 2025 (23 Jan Shift 2) JEE Main Previous Year Paper
Q58.A weak acid HA has degree of dissociation x . Which option gives the correct expression of ( pH pKa )? (1) 0 (2) log(1 + 2x) (3) log ( 1−xx ) (4) log ( 1−xx )
NCERT Chapters
- Class 11 Chemistry Ch 7: Equilibrium