05/25/15
CHEM 101 Exam 2
Name: __________________________________
Open-End Questions and Problems
Please read the following. To receive full credit for a question or a problem, in addition to the
correct answer, you must show a neat, complete, and logical method of solution where each
number is labeled with the appropriate unit and the final answer is rounded to the correct number
of significant digits. The correct answer without any work shown will generally get zero credit!
When an explanation is required, it should be brief, but accurate and complete. There are 8
questions for a total of 80 points. (Part of the exam with multiple choice questions is worth 70
points. Both parts combined are 150 points total.
1. (10 points) Fill the blanks in the following table.
0.500 mol CO2 + 0.300 mol O2 + 0.200 mol CO
The total number of molecules
The total number of atoms
The number of elements
The number of substances
The number of compounds
The number of elementary substances
The total mass in grams
(Show work!)
The total volume in liters (at room
temperature and normal pressure)
(Show work!)
2. (10 points) Fill the blanks in the following table.
Formula
Name
Number of Atoms
Number of Protons
Number of Electrons
Number of Valence
Electrons
Lewis Structure
(as a single structure or
as a set of all possible
resonance structures;
Dots & Dashes Style)
Number of Bonding
Electron Pairs
Number of Bonds
Number of Lone
Electron Pairs
Formal Charge on the
Central Atom
Oxidation State of the
Central Atom
Name of the Electron
Cloud Geometry
Name of the Geometric
Arrangement of Atoms
Drawing of the
Geometric Arrangement
of Atoms and Lone Pairs
around the Central Atom
Predicted Ideal Bond
Angle
Deviation from the Ideal
Bond Angle: yes or no?
Bonds Are of the Same
Length: yes or no?
ClO3−
NO3−
3. (10 points) Bromine pentafluoride is a covalent molecular compound.
(a) Predict the shape of the BrF5 molecule by drawing its Lewis structure and applying the
rules of VSEPR theory. Draw the spatial structure of the BrF5 molecule.
(b) Are the bonds within a BrF5 molecule polar or nonpolar? Explain.
(c) Are the molecules of BrF5 polar or nonpolar? Explain.
(d) Using the information sheet and the drawing from part (a), calculate the diameter of the
smallest possible spherical cavity that can host one entire BrF5 molecule. Show work.
[Hint: First calculate the covalent radii of the atoms present in BrF5.]
4. (10 points) By using the following data and by completing the diagram below, calculate the
2nd electron affinity of oxygen. Show work.
[Hint: Before placing the enthalpy values on the diagram, make sure that all the equations are
properly balanced by adding coefficients in front of atoms, molecules, ions, electrons, or
formula units. The enthalpy values given above are all per one mole of atoms, molecules, ions,
or formula units.]
1st electron affinity of oxygen
−141 kJ/mol
1st ionization energy for potassium
+419 kJ/mol
Bond energy of oxygen molecule
+495 kJ/mol
Lattice dissociation energy of K2O(s) (the energy required
to convert an ionic solid into a gas of its constituent ions)
+2238 kJ/mol
Enthalpy of formation of K2O(s)
−363.2 kJ/mol
Enthalpy of sublimation of potassium
+89.99 kJ/mol
Why do you think the 1st electron affinity of oxygen can be determined directly from an
experiment, but the 2nd electron affinity of oxygen can only be calculated using Hess’s law?
Briefly explain.
5. (10 points) Ozone, O3, absorbs ultraviolet radiation and dissociates into O2 molecules and O
atoms:
O3(g) + hv → O2(g) + O(g)
A 1.00 L sample of air at 22°C and 748 mmHg contains 0.25 ppm of O3. [1 ppm = one part
per million parts; it is similar to percent: 1% = one part per hundred parts.] How much energy,
in joules, must be absorbed if all the O3 molecules in the sample of air are to dissociate?
Assume that each photon absorbed causes one O3 molecule to dissociate, and that the
wavelength of the radiation is 254 nm.
6. (10 points)
(a) Using M as a generic symbol for an alkali metal, write chemical equations for two
different chemical reactions that are characteristic of all the alkali metals. Indicate
physical state of each reactant and product: (s), (l), (g), (aq).
(b) Using X as a generic symbol for a halogen, write chemical equations for two different
chemical reactions that are characteristic of all the halogens. Indicate physical state of
each reactant and product: (s), (l), (g), (aq).
(c) What is most common chemical property of the noble gases? Are there any compounds of
noble gases known? Give an example.
7. (10 points) In a standardization procedure, 13.76 mL of iron(II) sulfate solution were required
to reduce 25.00 mL of potassium dichromate solution, which was prepared by dissolving
1.692 g of K2Cr2O7 in water and diluting to 500.0 mL.
(a) Calculate the molarity of the potassium dichromate solution.
(b) Using the half-equation method, derive the balanced full-formula equation for of the
reaction used in titration experiment. [Fe(II) ion is oxidized to Fe(III) ion; dichromate ion
is reduced to chromium(III) ion. The reaction is performed in the presence of sulfuric
acid.]
(c) Using the balanced chemical equation from part (b), calculate the molarity of the iron(II)
sulfate solution.
8. (10 points) Place X in all appropriate boxes to indicate types of chemical bonds and
intermolecular forces responsible for the interactions between atoms, ions and molecules in
each sample of matter.
[Hint: First, classify the sample of matter as a pure substance or a mixture. Second, determine
which type of substance(s) is(are) present in the sample (ionic, covalent, or both) and what
particles are present there (molecules, ions, or molecules and ions). For molecules, draw
Lewis structures to predict their shapes and polarities.]