Calculate the pH of a titration of 50.00 mL of 0.100 M Phenylacetic acid , K a =

Question

Calculate the pH of a titration of 50.00 mL of 0.100 M Phenylacetic

acid , K a = 4.9 x 10 -5 , with 0.100 M NaOH at the following points:

SHOW ALL WORK IN NEAT DETAIL ON A SEPARATE PAGE .

(Be sure to write chemical equations and K a or K b expressions when

needed.)

a. (4 Pts) Before any NaOH is added.

b. (4 Pts) After 18.7 mL of NaOH are added.

c. (4 Pts) After 25.00 mL of NaOH are added.

d. (4 Pts) After 50.00 mL of NaOH are added.

e. (4 Pts) After 53.00 mL of NaOH are added.

f. (1 Pt) What is the pK a of the acid (show calculation)?

Explanation / Answer

1.

pH = 1/2[pKa+LOg C]
pKa = -log Ka = 5-log 4.9 = 4.31
=1/2[4.31 + log 0.1*50/1000]
= 1.0

2.

pH   = pKa+log [salt/acid]
   = 4.31+log [0.1*18.7/68.7/50*0.1/68.7]
     = 3.9

3.

pH   = pKa+log [salt/acid]
   = 4.31+log [0.1*25/75/50*0.1/75]
     = 4.0
4.
   pH = pKa+log [salt/acid]
   = 4.31+log [0.1*50/100/50*0.1/100]
    = 4.31
  
5.

pH   = 7+1/2[pKa+log C]
   =7+1/2[4.31+log 53*0.1/103]
   = 8.51

6.     pKa = -log Ka

= 5-log 4.9

= 4.31

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