Question

Which of the following acid-base indicators should be used for the titration of NH3 with HBr? Kb of ammonia is 1.8 ×10−5.

1. Methyl red, color change red/yellow at 4.4 < pH < 6.2 correct

2. Any of these is suitable.

3. None of these is suitable.

4. Phenolphthalein, color change colorless/red-violet 8.0 < pH <10.0

5. Neutral red, color change red/yellow 6.8 < pH < 8.0

Why is answer choice 1 correct?

Answer 1

In the titration of a weak base with a strong base, the equivalence point occurs on the acid side of 7.0. That tells you that 5,4,3, and 2 must be wrong and that the correct answer is #1. However, if you wish to calculate it that can be done, too. The pH at the equivalence point is determined by the salt, in this case NH4Br. The NH4^+ is hydrolyzed to
NH4^+ + H2O ==> H3O^+ + NH3
Ka = (Kw/Kb) = (H3O^+)(NH3)/(NH4^+)
You know Kw = 1E-14.
Kb = 1.8E-5 (for NH3)
(H3O^+) = x = (NH3)
(NH4^+) is about 0.05 if you titrate 0.1M HBr and 0.1M NH3. Work it out. I get (H^+) = 7.45E-6 and the H = about 5.13. That falls just about half-way between the limits of 4.4 and 6.2.