Name:
Chem 332
Analytical Chemistry
Exam II
Show all work for partial credit
1. ( 10 points) Find the pH of a solution prepared from 2.53 g of oxoacetic acid (C2H2O3,
Molar Mass = 74.04) 5.13 g of potassium oxoacetate ( C2HO3K, Molar mass 112.13)
and 103 g of water. You may assume the final volume of this solution is 109 mL.
(pKa oxoacetic acid = 3.46)
Moles oxoacetic acid =HA = 2.53/74.04 = .03417
Moles potassium oxoacetate = A- = 5.13/112.13 = .04575
pH = pKa + log [A-]/[HA]
= 3.46 + log [(.04575/.109)/(.03417/.109)]
= 3.46 + .13
= 3.59
2. (10 points) How many mLs of .113M HBr should be added to 52.2 mL of .0134M
morpholine (A base with a pKa = 8.49) to give a pH of 8.00?
The equation we want to use here is:
pH = pKa + log (B/HB+)
where 52.2 mL of morpholine = B
and you add HBr to B to convert it to HB+.
Mole morpholine = .0522 L x .0134M = 6.995 x10-4 moles
Setting up a reaction table
B
HB+
Morpholine + HBr 6 Hmorpholine+ + Br6.995x10-4
X
-X
-X
+X
So
8.00 = 8.49 + log (6.995x10-4 -X)/X
-.49 = log (6.995x10-4-X)/X
10-.49 = (6.995x10-4-X)/X
.324 =(6.995x10-4-X)/X ; .324X = 6.995x10-4-X; .324X + X = 6.995x10-4;
1.324X=6.995x10-4
X = moles of HBr = 5.28x10-4 moles
.113M = 5.28x10-4 mole/X L; X = 5.28x10-4 /.113 = 4.68x10-3L = 4.68 mL
3. (10 points) Often we use a pH meter to monitor the pH of a solution we are titrating.
When you do this, you typically record the volume of titrant added and the pH of the
solution after the addition. Explain three different ways to find the equivalence point
from this data.
1. Plot pH vs mLs of titrant. Look for the point with the steepest slope.
2. Plot the first derivative of the above plot. The first derivative will have a
maximum at the point with the steepest slope and that is the endpoint.
3. Plot the first derivative of the above plot (The second derivative of the first
plot) The crosses from positive to negative at the equivalence point.
4. Make a Gran plot. (Volume of base x [H+] vs volume of base) and extrapolate
the points before the equivalence point to 0.
4. (10 points) In our determination of iron we used the compound 1,10-Phenanthroline
to bind to the iron to make a colored adduct that we measured the absorbance of.
1,10-Phenanthroline is also a polyprotic species. Given that Phenanthroline has the
structure shown below in it’s fully protonanted form, and has pKa’s of 1.8 and 4.9,
A. Make a rough sketch the titration of Phenanthroline from a very low pH to a very high
pH. Make sure your sketch includes the pH at the initial point and at all equivalence
points.
I didn’t give a concentration so let’s just call it .1M
initial K = X2/.1-X
Changing pKa1 to Ka1 = 1.58x10-2
using the solver to solve the equation I find that X = [H+]=3.26x10-2, so my initial
pH is 1.49
The rough sketch should start at pH 1.49, it should flatten out to a buffer region
at pH 1.8, ther should be a small inflection at the first equivalence points, then it should
flatten out to a second buffer region at 4.9 then the pH should shoot up past say 8 or 9
at the second equivalence point.
B. Determine the principal species of phenanthroline at pH 1,3.5, and 6.
At pH 1
At pH 3.5
At pH 6
H2Phenanthroline
HPhenanthroline+
H2Phenanthroline2+
C. Finally, remembering that in our procedure we made sure the solution was buffered
at pH 3.5, predict the charged form for phenanthroline at this pH and take a guess why
this was important.
Phenanthroline must be in the Hphenanthroline+ state to properly bind to the Fe3+ ion to
make the colored complex.
5. What is the ionic strength of a solution that is 0.1M Na3PO4 and .05M Na2HPO4
[Na+ ] from Na3PO4 = .3M
[PO43-] from Na3PO4 = .1M
[Na+ ] from Na2HPO4 = .1M
[HPO42-] from Na2HPO4 = .05M
Ionic Strength =[ (.3+.1)x(+1)2 + .1(-3)2 + .05(-2)2]/2
=.75 M
6. (10 points) Write a charge balance equation that describes a solution that is 0.1M
Na3PO4 and .05M Na2HPO4
[Na+] + [H+] = 3@[PO43-] + 2@[HPO42-] + [OH-]
7. (10 points) Calculate the activity coefficients of H+ and I- in a solution that has an
ionic strength of .075M using the equation
H+ is 900 and the á for I- is 300.
For H+
Given that
Take home portion
Question 8 (12 points) . (Can be done with a spreadsheet)
Calculate the titration curve for 25 mL of .015M of NH3 being titrated with .01 M HCl.
The following the pH at the following points must be calculated and included in your
curve: 0, 10, 20, 30, 40 and 50 mL of titrant and the equivalence point.
(3 points) What is the most appropriate indicator for this titration?
Question 9 (15 points)
LiF has a Ksp of 1.7x10-3
Calculate the concentration of Li+ and F- in a saturated solution of LiF, ignoring
any activity effects.
Now calculate the activity coefficients for Li+ and F- and determine the
concentration of Li+ and F- in a saturated solution of LiF, when you include activity
effects.
Do successive approximations for this system until you come up with a final set
of concentrations for Li+ and F- that do not change.