UPSC Combined Geo-Scientist, Mains · 2024 · Mains Paper I
Q1(c). Choosing indicators
Question
Match the following titrations involving A: (i) Strong acid vs. weak base; (ii) Weak acid vs. strong base; (iii) Strong acid vs. strong base; (iv) Weak acid vs. weak base; (v) Redox reactions, with B: 1. Methylene blue (pKa = 3.8); 2. Bromothymol blue (pKa = 7.2); 3. No indicator; 4. Phenolphthalein (pKa = 9.3); 5. Methyl orange (pKa = 3.5).
Answer in brief
(i) – 5; (ii) – 4; (iii) – 2; (iv) – 3; (v) – 1.
Key points
- Strong acid – weak base has an acidic equivalence region: methyl orange is suitable.
- Weak acid – strong base has an alkaline equivalence region: use phenolphthalein.
- For strong acid – strong base, bromothymol blue changes near neutrality, within the steep pH jump.
- Weak acid – weak base normally lacks a sharp pH jump; no ordinary visual acid–base indicator gives a reliable end point.
- Methylene blue is assigned to redox reactions because its oxidised and reduced forms differ in colour.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
Coversacid-base indicatorsmethyl orangephenolphthaleinbromothymol bluemethylene bluechoice of indicator
All parts of Question 1
- 1(a) Minerals and their compositions5 marks
- 1(b) Magnetic moment of titanium(III)5 marks
- 1(c) Choosing indicators5 marks
- 1(d) Silica by volatilisation gravimetry5 marks
- 1(e) Spontaneity of Fe²⁺ oxidation5 marks
- 1(f) Oxidising power and charge transfer5 marks
- 1(g) pH dependence of cyanide masking5 marks
- 1(h) Terminal and bridging carbonyl bands5 marks
- 1(i) Identifying radioactive decay5 marks
- 1(j) Why ordinary lanthanoid carbonyls are unstable5 marks