Question

For the titration of 30.0 mL of 0.250 M acetic acid with 0.200 M sodium hydroxide, determine the pH when:

(a) 30.0 mL of base has been added.

(b) 37.5 mL of base has been added.

(c) 45.0 mL of base has been added.

(the answers must have the right significant figures)

Answer 1

pKa of acetic acid = 4.74

HA + NaOH ---------------> NaA- + H2O

30x0.25 0 0 0 initial mmoles

1) - 30x0.2 - - change

1.5 0 6 - after

buffer osolution . whose pH is given By Hendersen equation

pH = pKa + log [conjugate base]/[acid]

= 4.74 + log 6/1.5

=5.3420

b) - 37.5x 0.2 - - change

0 0 7.5 - after

The solution has only salt whose pH is given by

pH = 1/2[pKw +pKa +logC]

= 1/2 [ 14 +4.74 + log 7.5/67.5]

= 8.8928

c) - 45x0.2 - - change

0 1.5 7.5 - mmoles after reaction

the solution has a strong base and a salt

[OH-] = 1.5/75=0.02M

pH = 14 - pOH

= 14-[-log{OH-]}

= 12.3010