Please explain A38. Consider the following pH indicators and their pH range: Ind

Question

Please explain

A38. Consider the following pH indicators and their pH range: Indicator pH range for colour change Thymol blue Methyl red Bromothymol blue Phenolphthalein Alizarin yellow 1.2-3.0 4.8-6.0 6.0-7.6 8.2-10.0 10.1-12.0 Assuming an indicator works best when the equivalence point of a titration comes in the middle of the indicator range which of the acid - base indicators above would be the best to use for the titration of a 0.100 mol L- solution aniline (CsHsNH2) with a 0.100 mol L-1 solution HCI? (K, for CsHsNH2 3.8 x 10-10) (A) Thymol blue (B) Methyl red (C) Bromothymol blue (D) Phenolphthalein (E) Alizarin yellow

Explanation / Answer

Kb = 3.8*10^-10

meaning that the equivalence point will be approx:

Ka = Kw/Kb = (10^-14)/(3.8*10^-10) = 2.63*10^-5

in equilbirum

0.1 M --> 0.05 M

BH+ + H2O <-> B +´H3O+

Ka = [B][H3O+]/[BH+]

(2.63*10^-5) = x*x/(0.05-x)

x = 0.001133

pH = -log(0.001133) = 2.94

then, we need an indicator whic changes approx pH < 3

then, form the list, choose thymol blue

since it pH shoudl change once it achieves pH = 3

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