Download Chem 30A Final Exam

Chem 30A Final Exam
December 14, 2005
Show all work. Use reverse side for scratch paper calculations etc.
1. Draw valid Lewis structures for the simplest compounds of the second row elements
(except for Li) with flourine including lone pair electrons. Indicate the valence (i.e. # of
bonds), the central atom geometry, and the approximate bond angles in each case. Also
indicate when there is an exception to the octet rule, and whether the molecule is polar.
Central atom
Lewis dot
diagram
Molecule
Lewis structure
(w/bond angles)
the valence
VSEPR
geometry
octet rule
exception?
polar?
2. Gasoline is a mixture of many hydrocarbons. One of these hydrocarbons is octane.
a. Write a complete balanced equation for the complete burning of octane (C8H18).
b. Give the microscopic or particulate interpretation of the chemical equation in words.
c. Give the macroscopic interpretation of the chemical equation in words.
3. One set of reactants for rocket fuel is hydrazine, N2H4, and hydrogen peroxide, H2O2 which
react vigorously when mixed: N2H4(l) + 2H2O2(l)  N2(g) + 4H2O(g). A chemist mixes 2.0
mol N2H4 with 3.0 mol H2O2. What is the limiting reactant? How many moles of water
will be obtained?
4. Draw valid Lewis structures for the following compounds or ions. Indicate the VSEPR
geometry, approximate bond angles, and whether the molecule is polar. Draw more
than one resonance structure if appropriate.
a. SO2
b. H2S (hydrogen sulfide)
c. NO3- (nitrate ion)
d. Cl2CO (phosgene)
e. C2H2 (ethyne or acetylene)
f.
ClCN (cyanogen chloride)
g. CS2 (carbon disulfide)
h. PCl3 (phosphorous trichloride)
5. What is the molar volume of an ideal gas at 25oC? What is the molar volume at at 100oC?
6. When 4.01 g of mercury is strongly heated in air, the resulting oxide weighs 4.33 g. Calculate
the empirical formula of the oxide.
7. If steel wool (iron) is heated util it glows and is placed in a bottle containing pure oxygen, the
iron reacts spectacularly to produce iron(III) oxide.
a. Write a balanced chemical equation for the reaction.
b. If 1.25 g of iron is heated and placed in a bottle containing 0.0204 mol of oxygen gas,
what mass of iron(III) oxide is produced?
8. What is the density of gaseous carbon dioxide at 25oC and 1 atm? At STP?
9. The following molecule, called resveratrol, is found in red wine. Some scientists think this
compound is responsible for what is known as the “French Paradox”, i.e. the relatively low
incidence of heart disease in France even though the French eat lots of meat and dairy
products high in saturated fats.
a.
b.
c.
d.
draw in all the remaining implied hydrogens
draw in all the missing lone pairs of electrons
write the molecular formula for the compound
label each “central” atom with the appropriate geometry and bond angles.
HO
OH
OH
10. Which is lower in potential energy, an N2 molecule or two separated N atoms? Write a
reaction equation to show the formation of N2 from two gaseous N atoms. Is this reaction
exothermic or endothermic?
11. Identify the intermolecular forces present in each of the following compounds shown
below. Also explain the difference in boiling point between CF4 and CBr4.
Molecule
CH3-O-CH3
Boiling
Point
-25 oC
CF4
-128 oC
CBr4
189.5 oC
NF3
-129 oC
HF
19.5 oC
Intermolecular Forces
12. What mass of solid NaBr must be added to 3.00L of 0.100 M AgNO3 to precipitate all
of the Ag+ ions in the form of AgBr. What mass of AgBr will be formed? (Start with
a balanced chemical equation.)
13. For the reaction in the previous question write:
a. the complete ionic equation for the reaction, and
b. the net ionic equation for the reaction.
14. Write correct chemical formulas for the following compounds. In each case,
indicate whether the compound involves ionic bonding, covalent bonding, or both.
formula?
ionic?
covalent?
both?
potassium carbonate
ammonium fluoride
sodium oxide
chromium(III) oxide
selenium dioxide
arsenic triiodide
15. For each category write a balanced equation for your favorite reaction:
a. an acid dissociation
b. an acid – base neutralization
c. an oxidation – reduction (redox) reaction not involving oxygen.
d. a precipitation
e. a synthesis
f.
a decomposition
g. a combustion
16. You are titrating an unknown quantity of sulfuric acid (H2SO4) with a 0.1950 M NaOH
standard solution and you find it takes 32.50 mL to reach an endpoint (phenolpthalein color
change). What is the amount of sulfuric acid present in moles? Hint: Write a balanced
chemical reaction equation for the complete neutralization of the diprotic acid.
17. Extra: Determine [H+], pH, [OH-], and pOH for the following solutions. Hint: All of
the following are strong acids or strong bases.
Solution
1.4 x 10-3 M HClO4
[H+]
pH
[OH-]
pOH
3.0 x 10-5 M NaOH
5.0 x 10-2 M HNO3
0.0010 M KOH
18. Extra extra (use reverse side if necessary):
a. How do you plan to use your recently acquired knowledge of chemistry? And,
b. do you have any suggestions for improving this course?