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# Titrations

5) Using the following pKb (pKb = 4.11) of an unknown base, calculate the Ka of the corresponding conjugate acid and use the Ka value to identify the unknown base. Be careful to show all the steps used to convert pKb to Ka.
(See attachment for data)

6) Potassium hydrogen phthalate (KHP, K+OOC-C6H4-COOH) is a carboxylic acid which can be accurately weighed and yeilds solutions of stable pH. For this reason, KHP is used to standardize alkaline solution and to calibrate instruments capable of measuring pH.

25.00 mL of 0.7692 M KHP solution was titrated using 0.1000 M NaOH.

a) Calculate the pH of the KHP solution before the titration begins (Ka = 3.91 x 10^-6)
b) Calculate the pH of the NaOH solution before the titration begins.
c) Calculate the pH at the equivalence point.
d) Calculate the pH after the addition of 29.23 mL of NaOH solution.

#### Solution Preview

5. pKb = 4.11

pKa + pKb = 14

pKa = 14 - pKb = 14 - 4.11 = 9.89

-log(Ka) = 9.89

Ka = 10^(-9.89) = 1.29 x 10^(-10) = 1.3 x 10^(-10)

Hence it must be the conjugate base of phenol which is phenolate.

6. Let KHP be abbreviated as HA as it is a monoprotic acid.

a) HA -----> H+ + A-

Initial 0.7692M 0 0

Change -y +y +y

Equilibrium 0.7692 - y y y

Ka = [H+][A-] / [HA] = y^2 / (0.7692 - y) = 3.91 x 10^(-6)

y^2 = 3.00 x 10^(-6) - 3.91x10^(-6)y

Solving the quadratic equation ...

#### Solution Summary

Titrations are examined. The expert determines how to calibrate instruments which are capable of measuring pH is determined.

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