Buffer — Preparation, pH of buffer
Ionic Equilibrium
6
JEE Qs
8%
Hard
75
min
Master the mechanism of buffer action and correctly identify components (weak acid/base and its conjugate salt) for accurate pH calculations using the Henderson-Hasselbalch equation.
🧮 Key Formulas
✅ Key Points for JEE
- 1A buffer solution resists significant changes in pH upon addition of small amounts of acid or base.
- 2Acidic buffers are typically formed by a weak acid and its conjugate base (salt of weak acid and strong base). Basic buffers are formed by a weak base and its conjugate acid (salt of weak base and strong acid).
- 3Buffers can be prepared by mixing the weak acid/base with its salt, or by partial neutralization of a weak acid/base with a strong base/acid, respectively.
- 4The buffer capacity is maximum when [Weak Acid] = [Salt] or [Weak Base] = [Salt], at which point pH = pKa or pOH = pKb.
- 5Always use the initial moles or concentrations of the weak acid/base and its salt in the Henderson-Hasselbalch equation after considering any reactions or dilutions.
⚠️ Common Mistakes
- ✕Incorrectly identifying which component is the acid/base and which is the salt, especially when given chemical formulas.
- ✕Applying the Henderson-Hasselbalch equation to solutions that are not buffers (e.g., strong acid/base mixtures).
- ✕Failing to account for the change in moles of acid/base and salt when strong acid/base is added to the buffer, or when buffer is formed by partial neutralization.
- ✕Confusing pH = pKa + log([Base]/[Acid]) with pOH = pKb + log([Acid]/[Base]), leading to incorrect calculation for basic buffers.
- ✕Not considering dilution effects if mixing different volumes of solutions before applying the Henderson-Hasselbalch equation.
📝 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