# Problem: Methyl red has the following structure:It undergoes a color change from red to yellow as a solution gets more basic. Calculate an approximate pH range for which methyl red is useful. What is the color change and the pH at the color change when a weak acid is titrated with a strong base using methyl red as an indicator? What is the color change and the pH at the color change when a weak base is titrated with a strong acid using methyl red as an indicator? For which of these two types of titrations is methyl red a possible indicator?

###### FREE Expert Solution

General Reaction for an indicator:

HIn(aq) + H2O(l) ⇌ H3O+(aq) + In-(aq)

The pH range is as follow:

$\overline{){\mathbf{pH}}{\mathbf{=}}{{\mathbf{pK}}}_{{\mathbf{a}}}{\mathbf{±}}{\mathbf{1}}}$

Recall that pKa = -log Ka

${\mathbf{pH}}{\mathbf{=}}{\mathbf{-}}{\mathbf{log}}\left(5.0×{10}^{-6}\right){\mathbf{±}}{\mathbf{1}}\phantom{\rule{0ex}{0ex}}\phantom{\rule{0ex}{0ex}}{\mathbf{pH}}{\mathbf{=}}{\mathbf{5}}{\mathbf{.}}{\mathbf{3}}{\mathbf{±}}{\mathbf{1}}$

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###### Problem Details

Methyl red has the following structure:

It undergoes a color change from red to yellow as a solution gets more basic. Calculate an approximate pH range for which methyl red is useful. What is the color change and the pH at the color change when a weak acid is titrated with a strong base using methyl red as an indicator? What is the color change and the pH at the color change when a weak base is titrated with a strong acid using methyl red as an indicator? For which of these two types of titrations is methyl red a possible indicator?