Question

Phenolphthalein has a pka of 9.7 and is colorless in its acid form and pink in its basic form. part a for ph= 2.0 calculate [in−]/[hin].

Answer 1

Use the h-h equation!

pH = pKa + log[In-/HIn]
2.0 = 9.7 + log[In-/HIn]
-7.7 = log[In-/HIn]
10^-7.7 = [In-/HIn]
[In-/HIn] = 1.995 x 10^-8

Hope this helps!

Answer 2

1.9953 × 10⁻⁸

Step-by-step explanation:

Considering the Henderson- Hasselbalch equation for the calculation of the pH of the indicator as:

pH = pKa + log[In⁻]/[HIn]

Where Ka is the dissociation constant of the acid.

Given, pKa = 9.7

pH = 2.0

So,

2.0 = 9.7 + log[In⁻]/[HIn]

log[In⁻]/[HIn] = - 7.7

[In⁻]/[HIn] = Antilog (-7.7) = 1.9953 × 10⁻⁸