Answer to Question #213731 in Physical Chemistry for Tiffany

Question

The indicator methyl red has a pK_HIn = 4.95. It changes from red to yellow over the pH range from 4.4 to 6.2.

If the indicator is placed in a colourless buffer solution of pH = 4.55, what percent of the indicator will be present in the acid form, HIn?

Answer 1

K = 10^-4.95 = 1.122 ∙10^-5

1.122 ∙10^-5 = [A^-][H⁺] / [HA]

[H⁺] = 10^-4.55 = 2.818 ∙10^-5

[HA] = 1-[A^-]

1.122 ∙10^-5 = [A^-] ∙ (2.818 ∙10-5) / (1-[A^-])

0.398 = [A^-] / (1 - [A^-])

[A^-] = 0.28

[HA] = 1 - 0.28 = 0.72

C_%(HA) = 0.72