17. In a strong acid/weak base titration curve, the vertical portion around the

Question

17. In a strong acid/weak base titration curve, the vertical portion around the equivalence point is shorter than in a strong acid/strong base titration curve, so many indicators would not be suitable for accurately determining the equivalence point. Which one of the following indicators would be the best choice for the titration of 100.0 mL of 0.100 M methylamine, CH3NH2, with 0.100 M hydrochloric acid, HCl? Kb = 5.0 x 10^-4 for CH3NH2. a. methyl orange, red to yellow, pH 3.1-4.4 b. chlorophenol red, yellow to red, pH 4.8-6.4 c. alizarin yellow R, yellow to red, pH 10.1-12.0 d. indigo carmine, blue to yellow, pH 11.4-13.0

Explanation / Answer

At EP (equivalence point), the solution contains the conjugated acid CH3NH3+     

Ka = 1.10-14/Kb= 2x10-11

pKa = -log Ka = 11

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