A titration is to be performed to determine the concentration of an ammonia solu

Question

A titration is to be performed to determine the concentration of an ammonia solution using a 0.1 M solution of HCI. Two pH indicators arc available for you to use: thymol blue (pKa = 3.47) and phenolphthalein (pKa = 9.4). Which indicator do you select as being the more appropriate one to use? a. Both indicators would work equally well because at the equivalence point the pH of the solution changes very sharply from strongly basic to strongly acidic. b. Phenolphthalein is the better choice because it is one of the most commonly used pH indicators. c. Neither of these indicators would work because their pKa values are not close to 7.00, which is the pH of the solution at the equivalence point. d. Phenolphthalein is the better choice because the solution pH at the equivalence point will be close to the pKa value of 9.4. e. Thymol blue is the better choice because the solution pH at the equivalence point will be close to the pKt value of 3.47.

Explanation / Answer

The correct option is (e)

This is because the solution formed by mixing of strong acid and weak base has an acidic pH at equivalence point. So phenolphthalein would not work in that case since it is applicable for alkaline case only.

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