Answer:
The phenolphtalein is an acceptable indicator for the titration of KHP.
Explanation:
The acidic equilibrium for KHP is:
HP⁻ ⇄ P²⁻ + H⁺ Where ka=3,9x10⁻⁶
And ka is defined as: ka = [P²⁻] [H⁺] / [HP⁻] (1)
As molar concentration of the salt is 0,05M; [HP⁻] = 0,05M
The equivalence point will occurs when all HP⁻ reacts with NaOH to produce P²⁻ thus:
HP⁻ + NaOH → P²⁻ + H₂O + Na⁺
Using kw (1x10⁻¹⁴) it is possible to write:
P²⁻ + H₂O ⇄ OH⁻ + HP⁻;
kw/ka = [OH⁻] [HP⁻] / [P²⁻] (2)
The P²⁻ and H₂O will produce HP⁻ adn OH⁻until equilibrium, this could be represented as:
[P²⁻] = 0,05-x
[OH⁻] = x
[HP⁻] = x
Replacing in (2):
2,56x10⁻⁹ = x² / (0,05-x)
1,28x10⁻¹⁰ - 2,56x10⁻⁹x = x²
x² + 2,56x10⁻⁹x - 1,28x10⁻¹⁰ = 0
Solving for x:
x = -0.000011315 No physical sense, there are no negative concentrations
x = 0.0000113124 Real answer
Thus, [OH⁻] = 0,0000113124
As pOH = -log [OH⁻]
pOH = 4,95
As 14 = pH + pOH
pH = 9,05
As pH is in the pH range of phenolphtalein indicator, the phenolphtalein is an acceptable indicator for the titration of KHP.
I hope it helps!
