An acid–base indicator has Ka = 3.0 × 10−5. The acid form of the indicator is red and the basic form

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An acid–base indicator has Ka = 3.0 × 10−5. The acid form of the indicator is red and the basic form is blue. The [H+] required to change the indicator from 75% blue to 75% red is

Accepted Answer

$\underset{	ext{Red}}{HA} ightleftharpoons H^{+} + \underset{	ext{Blue}}{A^{-}}$$\left\lbrack H^{+} ightbrack = \frac{K_{a}.\lbrack HAbrack}{\left\lbrack A^{-} ightbrack} $$$	herefore\left\lbrack H^{+} ightbrack_{	ext{Required}} = \left\lbrack H^{+} ightbrack_{2} - \left\lbrack H^{+} ightbrack_{1} = K_{a}\left( \frac{75}{25} - \frac{25}{75} ight) = 8 	imes 10^{- 5}	ext{ M}$$

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