UPSC Combined Geo-Scientist, Mains · 2021 · Mains Paper I
Q6(b). Methyl-red colour and ionisation
Question
Methyl red has Ka = 10⁻⁵. The acid form HIn is red and its conjugate base, In⁻ is yellow. Complete the following table for pH 3, 5 and 7: [In⁻]/[HIn] and colour. [blank table given in the paper]
Answer in brief
At pH 3, 5 and 7 the ratios are 0.01, 1 and 100; the respective colours are red, orange and yellow.
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Coversmethyl redacid-base indicatorHenderson-Hasselbalch equationindicator colour changepKa
All parts of Question 6
- 6(a) Acid required by an equimolar carbonate mixture10 marks
- 6(b) Methyl-red colour and ionisation10 marks
- 6(c) Weak diprotic base titrated with strong acid10 marks