Exam 3: Chapters 9-12
Complete five (5) of the following problems.
Each problem is worth 16 points.
the problems you do not want graded.
You must show your work to receive credit for problems
Report your answers with the appropriate number of significant figures.
Consider the titration curve below.
The curve is the result of the titration of 20.00 mL of a
0.100 M solution of one of the acids below with 0.100 M NaOH.
Identify the acid.
justify your decision.
2.17, 7.20, 12.15
Volume NaOH Delivered (mL)
First, let’s look at the experimental conditions.
Since the concentration of the acid and base
are the same, we’d expect to see an equivalence point at 20 mL if the acid is monoprotic, 20
and 40 mL if the acid is diprotic (and the K
’s are different enough), and at 20, 40, and 60
mL if the acid is triprotic (and the K
’s are different enough).
We see a single clear
equivalence point at 40 mL, therefore the acid cannot be monoprotic, ruling out acetic,
pyruvic, and HCl.
The remaining options are polyprotic acids.
What would we expect from
differ by ~10
, we’d expect to see a clear distinction
between the two equivalence points (@ 20 and 40 mL).
Since we don’t observe this, the
acid isn’t phospohoric.
The same argument holds in this case as well.
Because the K
’s are quite
different, we’d expect to see both equivalence points, since we do not, the acid can’t be
’s are quite similar, therefore we would not expect to see two distinct
Instead, we’d expect to see them merge at ~40 mL.
This is what we
observe in the titration curve.
Therefore, the acid must be
You could do
additional calculations to verify this, but it is unnecessary.