Chemistry 150
Exam II
Winter Quarter 2002
1. (6) Compare and contrast the bonding in Na2O vs. that in H2O. Clearly show how the
bond is formed in each case.
2. (4) Which compound would you expect to have the higher melting point, MgCl2 or
CaCl2? Explain.
3. (8) The following is a modified diagram from your textbook showing the changes in
potential energy as two hydrogen atoms approach each other to form the hydrogen
molecule, H2. Based on this diagram:
a. What is the approximate bond length in H2? Explain your answer with
reference to the diagram below.
b. What is the approximate bond energy in H2? Explain your answer with
reference to the diagram below.
4. (16) Draw the optimal Lewis structure (showing all lone pairs and formal charges) for
each of the following.
a. ClO2b. SO3
c. SF4
d. HNO2
5. (6) Give the predicted geometry for compounds a-c above. You can either give the
correct name, or draw a suitably representative 3-dimensional picture.
6. (4) Of the compounds in question #4, which would be polar? Explain.
7. (8) Compare the geometry and the bond angles in the molecules SO2 and H2O.
8. (6) Draw all possible isomers of the compound C3H8O.
9. (4) What is the difference between a resonance structure and an isomer? Explain.
10. (15) Carbonic acid, H2CO3, is a weak acid that can lose one or both of its hydrogens
to form either the bicarbonate ion, HCO3- , or the carbonate ion CO32-.
a. Draw a Lewis structure and all equivalent resonance structures for each of
these species, and determine the carbon-oxygen bond order in each case.
b. In which compound are the carbon-oxygen bonds the longest? The shortest?
Explain.
c. In which compound are the carbon-oxygen bonds the strongest? The weakest?
Explain.
11. (8) The FTIR spectrum of C4H10O is shown below. Draw a structure for this
compound that is consistent with its spectrum. (there may be several possible
acceptable answers). Explain your reasoning. After you have drawn the structure,
identify the geometry at every central atom.
11. (15) The following reactions show the sequence of events that takes place during the
titration of a sample containing ascorbic acid (Vitamin C). First, I3- is generated by
the reaction of KIO3 with KI. Then, the I3- reacts with the Vitamin C. Finally, the
excess I3- is titrated with a solution of thiosulfate.
6 H+(aq) + KIO3 (aq) + 8KI (s) ------------> 9 K+ (aq) + 3 I3- (aq) + 3H2O(l)
I3- + C6H8O6 --------> C6H6O6 + 3 I- + 2H+
I3- + 2 S2O32- -------> 3 I- + S4O62Data for one such experiment is shown below.
Molarity of S2O32Molarity of KIO3
Volume of KIO3 used
Mass of KI used
Volume of S2O32- used to titrate
excess I3Volume of sample containing
Vitamin C
0.0785 M
0.0250 M
25.00 mL
2.00 g
36.55 mL
25.00 mL
Use the data above to calculate the mg of Vitamin C in the unknown sample. Show all
your work, including the calculation necessary to identify the limiting reactant in
reaction (1).
What is a potential problem that could arise in this experiment if the sample to be titrated
contains a very large amount of Vitamin C? Explain.