Exam_2_2010_answer_2_key - Problem 1(16 pts Fractional...

Problem # 1 (16 pts): Fractional Composition Diagrams
A fractional composition diagram is plotted (below) for o-phosphoric acid.
(a) (4 pts) Write the pertinent acid-dissociation reaction equations in the full pH
range.
H
3
PO
4
+ H
2
O ↔ H
3
O + + H
2
PO
4
-
H
2
PO
4
-
+ H
2
O ↔ H
3
O
+
+ HPO
4
2-
HPO
4
2- + H
2
O ↔ H
3
O + + PO
4
3-
(b)
(4 pts) Give the approximate K
a
values (not pKa’s) as read from the plot.
10
-2.15
= 7.1 x 10
-3
These are the actual values.
10 -7.20 = 6.3 x 10 -8
10 -12.15 = 7.1 x 10 -13
(c) (4 pts) What is the principal species at pH = 14.00?
PO
4
3- or A 3-
(d) (4 pts) What is the principal species at pH = 0.00?
H
3
PO
4
or H
3
A
pH
0 2 4 6 8 10 12 14
α
(fraction of each form)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Fractional composition diagram for o-phosphoric acid

Problem # 2 (18 pts): Redox Titrations
100 mL of 0.100 M Fe 2+ was titrated with 0.100 M Ce 4+ in 1.00 M HNO
3
. Formal
potentials for Fe
3+
/Fe
2+
and Ce
4+
/Ce
3+
(in 1.00 M HNO
3
) are 0.746 V and 1.61 V.
(1) (6 pts) What was the potential at the equivalence point? Derive the expression
you use to calculate this value from Nernst equations. (If you can’t derive it, you will get
partial credit for the short-cut equation.)
E
eq
= ½( E
Ce
o + E
Fe
o ) = ½ (1.61 + 0.746 ) = 1.18
(2)
(6 pts)What was the equilibrium concentration of Ce 4+ after 50 mL of the titrant
was added? Based on this calculation, was the “x = 0” approximation valid (one sentence