which of the followingindicators would be the best to use when 0.050 M benzoic a

Question

which of the followingindicators would be the best to use when 0.050 M benzoic acid(Ka=6.6 x 10^-5) is titrated with 0.05 M NaOH? a) bromphenol blue, pH range:3.0-4.5 b) bromcresol green, pH range 3.8-5.4 c) alizarin, pH range: 5.7-7.2 d) phenol red, pH range; 6.9-8.2 which of the followingindicators would be the best to use when 0.050 M benzoic acid(Ka=6.6 x 10^-5) is titrated with 0.05 M NaOH? a) bromphenol blue, pH range:3.0-4.5 b) bromcresol green, pH range 3.8-5.4 c) alizarin, pH range: 5.7-7.2 d) phenol red, pH range; 6.9-8.2

Explanation / Answer

Let us calculate pH at equivalence point

benzoate formed = 0.05/2 = 0.025 M

benzoate + H2O <==> benzoic acid + OH-

let x amount hydrolyzed

Kb = 1 x 10^-14/6.6 x 10^-5 = x^2/0.025

x = [OH-] = 1.95 x 10^-6 M

[H3O+] = 1 x 10^-14/1.95 x 10^-6 = 5.14 x 10^-9 M

pH = -log[H3O+] = -log(5.14 x 10^-9) = 8.3

The bets pH would be which shows color change neat equivalence point of titration

Therefore, here the best option would be,

d) Phenol red.

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