CHM 134 General Chemistry I
Exam 3 Fall 2005
Name
KEY
Section I: Each question in this section is worth 2 points.
1. What is the mass of one mole of sodium hydroxide?
40.00 g
2. A silicon atom has 4 valence electrons.
3. Which combination of atoms is most likely to produce a compound with ionic bonds?
a) Al and F
4. How many grams are in 0.0751 mol Br2?
e) 12.0 g
5. In molecules, as bond order increases,
e) bond length decreases and bond energy increases.
6. Predict which of the following compounds will have the most polar covalent bond(s).
a) HF
7. What is the molecular geometry around a central atom that is sp3 hybridized and has
one lone electron-pair?
d) trigonal-pyramidal
8. What is the mass percent of chlorine in FeCl2?
d) 55.9%
9. Which of the underlined atoms (C1, C2, N, and O) are sp2 hybridized?
..
a) C1 and C2
OH
H
H
..
.O
.
C1
H
H
C
C
NH2 H
C C2
C
C
C
H
H
C
H
3
10. What is the molecular geometry around a central atom that is sp d hybridized and has
three sigma bonds and two lone electron-pairs?
e) T-shaped
11. How many moles are in 35.7 g of copper (II) sulfate?
b) 0.224 mol
12. Which of the following are possible Lewis structures for C2H6O?
H
H
H C H
H
..
C O
..
H
H
..
H C O
.. C H
H
H
H
H H
(2)
(1)
..
H C C O
.. H
H H
(3)
d) 2 and 3
13. According to VSEPR theory, the electron-pair geometry of ICl3 is trigonal
bipyramidal and its molecular geometry is T-shaped.
14. What is the formal charge on each atom in a hypobromite ion, OBr-?
d) O = -1, Br = 0
15. Which one of the following molecules has a dipole moment (is polar)?
c) NCl3
16. If 1.00 g of an unknown molecular compound contains 4.55 × 1021 molecules, what is
its molar mass?
e) 132 g/mol
17. Which of the following are resonance structures for the formate ion, HCO2-?
..
O
.. .
C O
.. .
H
(1)
c) 3 and 4
-
..
O
..
C O..
H
(2)
-
..
O
.. .
C O
.. .
H
(3)
-
..
O
..
C O..
H
(4)
-
18. How many sigma (σ) bonds and pi (π) bonds are in the following molecule?
H
H
C
C
H
C
C
H
e) seven σ and three π
19. One product of the combustion of ethylene, C2H4, is carbon dioxide. What change in
hybridization of the carbon occurs in this reaction?
H
H
C
C
H
O
C
O
H
ethylene
carbon dioxide
e) sp2 to sp
20. How many oxygen atoms are in 1.50 grams of SO3? 3.38 x 1022 O atoms
Section II: Show your work to solve each question.
21. (4 points) The density of carbon tetrachloride is 1.59 g/mL. How many chlorine
atoms are present in 55.0 mL of carbon tetrachloride? 1.37 x 1024 Cl atoms
22. (4 points) A molecule is found to contain 47.35% C, 10.60% H, and 42.05% O. What
is the empirical formula for this molecule?
The formula is C3H8O2
23. (14 points) Draw a molecular orbital diagram for the diatomic molecule, O2. Label
all of the energy levels (both occupied and unoccupied). Show the population of each
level with arrows. Use the diagram to answer the following questions. Make sure
you explain your answers.
What is the bond order of O2?
2
Is O2 diamagnetic or paramagnetic?
Paramagnetic, since there are unpaired
electrons.
Which species has the shortest bond length (strongest bond)?
a) O22+
Since you are removing electrons from an antibonding orbital, the
bond strengthens.
24. (4 points) Draw the five possible resonance structures of the phosphate ion, PO43-. In
each case the phosphorus atom is the central atom of the molecule.
3-
3-
O
O
3-
O
P
O
O
O
P
O
3-
O
O
O
P
3-
O
O
O
O
O
O
P
O
O
P
O
25. (4 points) Draw an appropriate Lewis dot structure for each molecule:
(CH3)(CH2)(CO)(OH)
Cl2CO
O
H
C
Cl
Cl
H
H
O
C
C
C
H
H
O
H
O
Section IV: Complete the table below. Each blank is worth 2 points. (30 points total)
Formula
Lewis Structure
Geometry
Hybridization on central
atom
see-saw
sp3d
trigonal pyramidal
sp3
trigonal
bipyramidal
sp3d
tetrahedral
sp3
linear
sp
F
F
SF4
F
S
F
1+
H3O1+
H
O
H
H
F
F
PF5
F
P
F
F
1-
O
ClO41-
O
Cl
O
O
CS2
S
C
S