A Titration is performed in which .05 M NaOH is added to 10 mL of Arsenic Acid
(H3AsO4).
H3AsO4 + H2O
H2AsO4- + H2O
HAsO42- + H2O
H3O+ + H2AsO4H3O + + HAsO42H3O + + AsO43-
Ka=2.5*10-4
Ka=5.6*10-8
Ka=3.0*10-13
a) Sketch the general shape of the titration curve for arsenic acid with aqueous sodium
hydroxide. You do not need to label any specific values.
b) Arsenic acid is triprotic. The equivalence points occur at 12, 24, and 36 mL of added
NaOH. Add this information to the graph.
c) Find the pH before the base is added.
d) What substance(s) is/are present in the largest concentration when 45 mL of base is
present, and of what nature is this? Ignoring the spectator ions, what substance(s) is/are
present when 35 mL of base is added?
e) Calculate the pH of the solution when 7.2 mL of NaOH is added
f) What is the pH at the second equivalence point?
g) What is the pH of the solution when 30 mL of NaOH is added?
h) You decide to empty the entire 50 mL bottle of NaOH into the flask. Find the pH.
i) The titration experiment has been slightly altered. Pretend that the equivalence points
and the volumes remain the same, but the concentration of the acid changes to .1 M. How
is the pH at the second equivalence point different than the pH of the original experiment
at the second equivalence point?
j) When 12 mL of NaOH is added to the flask, what substance is present in the highest
concentration? Of what nature is it? Calculate its concentration.