Indicators — pH range, selection
Ionic Equilibrium
6
JEE Qs
8%
Hard
60
min
Always match the indicator's pH transition range to the pH at the equivalence point of the titration to ensure accurate results.
🧮 Key Formulas
✅ Key Points for JEE
- 1Acid-base indicators are weak organic acids or bases whose conjugate forms have different colors.
- 2The color change of an indicator occurs over a specific pH range, not at a single pH point, typically when the ratio of the two colored forms ([In-]/[HIn] or [In+]/[InOH]) changes from approximately 1/10 to 10/1.
- 3The effective pH range for an indicator's color change is approximately pKa_indicator +/- 1 (or pKb_indicator +/- 1 for basic indicators, then converted to pH).
- 4For accurate titration, an indicator must be chosen such that its pH transition range falls entirely within the sharpest, steepest portion of the titration curve, which encompasses the equivalence point pH.
- 5Different types of acid-base titrations (strong acid-strong base, strong acid-weak base, weak acid-strong base) have different equivalence point pH values, thus requiring different indicators.
⚠️ Common Mistakes
- ✕Incorrectly assuming an indicator changes color exactly at its pKa_indicator value, rather than over a range.
- ✕Selecting an indicator based solely on its visible color without considering the specific pH at the equivalence point of the titration being performed.
- ✕Failing to understand that the indicator itself is a weak acid/base and its concentration must be negligible to avoid affecting the titration.
📝 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