You Have a Summer Job as an Intern in an Analytical

question 147

Multiple Choice

You have a summer job as an intern in an analytical chemistry laboratory. You are about to titrate a solution of nicotine (pK_b = 6.0) , which is a base similar to ammonia, with a standard hydrochloric acid solution. Two pH indicators are available for you to use: thymol blue (pK_a = 3.47) and phenolphthalein (pK_b = 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 pK_a 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 sample pH at the equivalence point will be close to the pK_a value of 9.4.
• E) Thymol blue is the better choice because the sample pH at the equivalence point will be close to the pK_a value of 3.47.

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