Q19.124CP - Chemistry: Molecular Nature Of Matter And Change

Question

The acid-base indicator ethyl orange turns from red to yellow over the pH  range 3.4  to 4.8 . Estimate Ka for ethyl orange.

Step-by-Step Solution

Answer

The estimation is $\mathrm{Ka}=7.9\times {10}^{-5}$ for ethyl.

Explanation of the Concept

Acid-base indicators are indicators that are used to determine the concentration of an acid in a solution. They are usually coloured organic dyes that are either weak acid or base. In aqueous solution, they form an equilibrium with their conjugate base or acid, respectively.

Estimation Ka for ethyl

We can presume that the indicator's central value is value   range of transition In this instance, because the range is from 3.4 to 4.8, we can presume the  value is the central value, which can be calculated using the following equation:

 $\frac{3.4+4.8}{2}=4.1$

We only need to calculate an estimate Ka for ethyl orange now:

$\mathrm{Ka}={10}^{-\mathrm{pKa}}={10}^{-4.1}\mathrm{Ka}=7.9\times {10}^{-5}$ 

Hence, the estimation is $\mathrm{Ka}=7.9\times {10}^{-5}$
 for ethyl orange.