Question

You determine that it takes 18.66 mL of base to neutralize a 25.00 mL sample of your unknown acid solution. The pH of the solution when about 9.33 mL of base had been added was 2.90. You notice that the concentration of the base was 0.1232 M.

a. What are the Ka and the pKa of this unknown acid?

b. What is a possible identity of this unknown acid?

c. What is the concentration of the unknown acid?

Answer 1

V = 18.66 mL of base , M = 0.1232 M of base

V = 25 mL of acid, M = ?

pH = 2.90 when V base = 9.33 mL

a)

mmol of base required for neutralization = 18.66*0.1232 = 2.298912 mmol of base used

mmol of acid = mmol of base = 2.298912

Macid = mmol/Vacid = 2.298912/25 = 0.09195648 M

now...

for Ka/pKa:

use buffer equations

pH = pKa + log(conjugate/acid)

mmol of acid initially = 2.298912

mmol of base initially = 0

after adding --> 9.33 mL of 0.1232 M --> mmol of base = 9.33 *0.1232 M -->1.149456 mmol of base

so...

conjugate base formed = 1.149456

weak acid left = 2.298912-1.149456 = 1.149456

so

pH = pKa +log(conjugate base / weak acid)

pH = pKa + log(1.149456/1.149456)

pH = pKa

2.9 = pKa

Ka = 10^-pKA = 10^-2.90 = 1.25*10^-3

b)

nearest pKa value is 2.85, which is Chloroacetic acid

c)

concentration of unkown acid:

Macid = mmol/Vacid = 2.298912/25 = 0.09195648 M