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Phenolphhalein indicator is used in this experiment because it changes color in the pH range of...

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Phenolphhalein indicator is used in this experiment because itchanges color in the pH range of 8.0 to 9.6.
Show by calculation that phenolphthalein is an acceptableindicator for the titration of KHP with NaOH.
This is true if the pH at the equivalence point falls withinthis range.
In essence, what you need to do is to calculate the pH of theunprotonated phthalate salt. You may assume that the concentrationof the salt is approximately 0.05 M at the equivalance point.
HINT: The salt is derived from the weak diprotic acid, which hasKa1 = 1.2 * 10^-3 and Ka2 = 3.9 * 10^-6

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4.4 Ratings (820 Votes)

assume salt is Na2P

P-2 + H2O ----------------------> HP-Â Â + OH-

0.05Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 0Â Â Â Â Â Â Â Â Â Â 0 ------------------> initial

0.05-xÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â xÂ Â Â Â Â Â Â Â Â Â x --------------------> equilibrium

Kb1 = [HP-][OH-]/[P-2]

Kw / Ka2 = x^2 / 0.05-x

1.0 x 10^-14 / 3.9 x 10^-6 = x^2 / 0.05-x

2.56 x 10^-9 = x^2 / 0.05-x

x^2 + 2.56 x 10^-9 x - 1.28 x 10^-10 = 0

x = 1.13 x 10^-5

[OH-] = x = 1.13 x 10^-5 M

pOH = -log [OH-]

pOH = -log (1.13 x 10^-5)

pOH = 4.95

pH + pOH = 14

pH = 9.05

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