Chapter U6.123, Problem 7E

Interpretation Introduction

Interpretation: pH at which thymolphthalein changes color needs to be explained.

Concept Introduction: The color change for an indicator occurs when the ${\text{H}}^{\text{+}}$ ion concentration is equal to K. This happens when the concentration of HIn molecules is equal to the concentration of the ${\text{In}}^{-}$ ions.

Answer to Problem 7E

Thymolphthalein changes color at pH 10.

Explanation of Solution

Color changes for an indicator is occurs if ${\text{H}}^{\text{+}}$ ion concentration is equal to constant K. Given,

  ${\text{K}}_{\text{a}}={10}^{-10}$

Since, pH is the negative logarithm of ${\text{H}}^{\text{+}}$ ion concentration. Therefore, substitute the above values.

  $\begin{array}{c}\text{pH}=10\log 10\\ =10\end{array}$

  $\begin{array}{c}\text{pH}=10\log 10\\ =10\end{array}$

Therefore, pH is 10.

Conclusion

Thymolphthalein changes color at pH 10.