Question

the acid-base indicator HIn shows a colour change at ph -6.4 when 20 percent of it is ionised the dissociation constant of the indicator is​

Answer 1

Answer:

1.99×$10^{-8}$

Concept:

The degree of dissociation is the measure of amount dissociated as ions in the solvent to generate electrolytic solution.

Strong acid and strong base fully dissociate in the solute so it's degree of dissociation is 100%.

While weak acids and bases are partially dissociates into ions having small dissociation constant.

pH: It is a measure of acidity or basicity of  the solutions.

pH range varies from 0-14.

Acids are having pH range from 0 to 7 and bases are from 7 to 14.

Neutral solutions are at pH 7.

Lower the pH more the acidity and as result of increase in pH basicity of the solution increases.

Degree of dissociation is the % of the amount dissociated into ions.

Formula :

1. $K_{a}$ = C × ($degree of dissociation^{2}$)/ 1 - degree of dissociation

      where,

     $K_{a}$ = dissociation constant

     C = concentration

   2. pH = - log [ H+]

      where [ H+] is the concentration of the ions

Calculation:

Calculation of [H+]:

given ,pH = 6.4

∴ pH  = - log [ H+]

 6.4 = - log [ H+]

∴[H+] = 3.98 ×$10^{-7}$ = C

Calculation of dissociation constant,

Given,

20% of indicator is dissociated...

∴ degree of dissociation = 20/ 100 = 0.2

from the formula ,

dissociation constant $K_{a}$ = C ×$dissociation constant^{2}$ / 1- dissociation         constant

∴$K_{a}$ = 3.98 ×$10^{-7}$ ×$0.2^{2}$ / 1- 0.2

∴ $K_{a}$ = 1.99 ×$10^{-8}$

# SPJ3