Chem. 1C Midterm 1
Version B
April 20, 2016
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of last name
Name:__________________________________________
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Perm Number:___________________________________
All work must be shown on the exam for partial credit. Points will be taken off for
incorrect or no units. Calculators are allowed. Cell phones may not be used for calculators.
On fundamental and short answer problems you must show your work in order to receive
credit for the problem. If your cell phone goes off during the exam you will have your
exam removed from you.
Fundamentals
(of 36 possible)
(
)+(
)=
Problem 1
(of 16 possible)
Problem 2
(of 18 possible)
Multiple Choice
(of 30 possible)
Midterm Total
(of 100 possible)
1
Fundamental Questions
Each of these fundamental chemistry questions is worth 6 points. You must show work to
get credit. Little to no partial credit will be rewarded. Make sure to include the correct
units on your answers.
1)
6 pts
At normal temperatures and pressures a certain metal element M forms a
crystal with an fcc unit cell and lattice constant a =408 pm. The density of
is measured to be 19.30𝑐𝑚𝑔 3 . Using only this information, identify the
element M and write its chemical symbol below.
Faced centered cubic crystals have 4 atoms per unit cell.
𝑚
𝑑=𝑉
If we calculate the volume of the unit cell we will be able to
calculate the mass of the 4 atoms.
Calculate the volume of the unit cell
𝑚
𝑑=
𝑉
𝑚
19.30 𝑐𝑚𝑔 3 = (4.08×10−8 𝑐𝑚)3
𝑚 = 1.31 × 10−21 𝑔
Calculate the mass of 1 atom
𝑚
1.31×10−21 𝑔
=
= 3.28 × 10−22 𝑔
4
4
Calculate molar mass
1
𝑔
𝑚𝑁𝑎 = (3.28 × 10−22 𝑔)(6.022 × 1023 𝑚𝑜𝑙
) = 197 𝑚𝑜𝑙
Therefore, M is Au
2)
6 pts
Use the LE model to describe the bonding in CO32-.
The carbon atom forms one 𝜎 bond to each of the
single bonded oxygen atoms. This bond is
formed from the overlap of a sp2 hybridized
orbital on the carbon atom and a sp3 hybridized
orbital on the single bonded oxygen atoms. A
third 𝜎 bond is formed from the overlap of an
sp2 hybridized orbital on the carbon atom and a sp2 hybridized orbital on
the double bonded oxygen atom. The π bond between the double bonded
oxygen atom and the carbon atom is formed from the overlap of the
unhybridized p orbitals on both the carbon and oxygen atoms. The loan
pair electrons on the double bonded carbon sit in sp2 hybridized orbitals
and the loan pair electron on the single bonded oxygen atoms sit in sp3
hybridized orbitals.
3)
6 pts
What is the complete (valence and core) electron configuration for O22+ and
is it paramagnetic or diamagnetic?
O22+ (14 e-)
(σ1s)2(σ1s*)2(σ2s)2(σ2s*)2(σ2p)2(π2p)4
Diamagnetic
2
4)
6 pts
Sketch the molecular orbital and label its type (σ or π; bonding or
antibonding) that would be formed when the following atomic orbitals
overlap.
+
π
+
5)
6 pts
σ*
What intermolecular forces are present in:
Circle the strongest intermolecular force present
KBr
Ionic
CH3(CH2)50CH2OH
CO2
H-Bond, Dipole-Dipole, London
London
OBr2 Dipole-Dipole, London
6)
6 pts
Consider the phase diagram for iodine shown here
What is the normal boiling point for iodine? 184.4°C
What is the melting point for iodine at 1 atm?113.6°C
What state is present at room temperature and normal atmospheric
pressure? Solid
What state is present at 186°C and 1.0 atm? Gas
3
Short Answer Questions
Each of the following short answer questions are worth the noted points. Partial credit will
be given. You must show your work to get credit. Make sure to include proper units on
your answer.
1)
16 pts Butane (C4H10) has a heat of vaporization of 22.44 𝑘𝐽 and a normal boiling
𝑚𝑜𝑙
point of -0.4˚C. A 250 mL sealed flask contains 0.55 g of butane at -22˚C.
What percentage (in moles) of butane is in the liquid phase?
In order to find the amount of the liquid that is in the gas phase you need to
find the vapor pressure at -22˚C.
𝑃𝑣𝑎𝑝1
∆𝐻𝑣𝑎𝑝 1
1
𝑙𝑛 (
)=
( − )
𝑃𝑣𝑎𝑝2
𝑅
𝑇2 𝑇1
Since the normal boiling point is -0.4˚C (272.75 K), when the temperature is
272.75 K the vapor pressure is 1 atm.
𝑘𝐽
22.44 𝑚𝑜𝑙
1 𝑎𝑡𝑚
1
1
𝑙𝑛 (
)=
(
−
)
𝑘𝐽
𝑥
0.083145 𝑚𝑜𝑙∙𝐾 251.15 𝐾 272.75 𝐾
x = 0.427 atm
Determine the number of moles of gas the correspond to P=0.427 atm,
V=250 mL, and T = 251.15 K
(0.427 𝑎𝑡𝑚)(0.250 𝐿)
𝑃𝑉
𝑛=
=
= 0.00518 𝑚𝑜𝑙
𝐿∙𝑎𝑡𝑚
𝑅𝑇 (0.08206 𝑚𝑜𝑙∙𝐾
)(251.15 𝐾)
Determine total moles of butane
1 𝑚𝑜𝑙 𝐶 𝐻
0.55 𝑔 𝐶4 𝐻10 ( 58.1234 𝑔10 ) = 0.0095 𝑚𝑜𝑙
Determine moles of liquid left in the container
0.0095 𝑚𝑜𝑙 − 0.00518 𝑚𝑜𝑙 = 0.0043 𝑚𝑜𝑙
Determine percentage of butane in liquid phase
0.0043 𝑚𝑜𝑙
100%
= 45%
0.0095 𝑚𝑜𝑙
4
2)
2a)
Predict the molecular structure and the bond angle for each of the
following.
You must draw the Lewis structures of each to receive full credit.
3 pts
XeCl2
XeCl2
Valence e-
Xe
8
2(Cl)
2(7)
Angle: 180˚
Shape: Linear
2b) 3 pts
ICl3
ICl3
Valence e-
I
7
3(Cl)
3(7)
3 pts
TeF4
TeF4
Valence e-
Te
6
4(F)
4(7)
Total
28
Angles: <90˚ and <120˚
Shape: Sea-Saw
2d) 3 pts
Total
28
Angles: <90˚
Shape: T-Shaped
2c)
Total
22
PCl5
PCl5
Valence e-
P
5
5(Cl)
5(7)
Total
40
Shape: Trigonal Bipyramidal Angle: 90˚ and 120˚
2e) 6 pts
Which of the molecules in a-d have net dipole moments (are polar)?
ICl3 and TeF4 have net dipole moments making them polar.
5
Multiple Choice Questions
On the ParScore form you need to fill in your answers, perm
number, test version, and name. Failure to do any of these things will
result in the loss of 1 point. Your perm number is placed and
bubbled in under the “ID number.” Do not skip boxes or put in a
hyphen; unused boxes should be left blank. Bubble in your test
version (B) under the “test form.” Note: Your ParScore forms will
not be returned to you, therefore, for your records, you may want to
mark your answers on this sheet. Each multiple choice question is
worth 5 points.
1. Which of the following statements is correct?
A) Free rotation may occur about a double bond.
B)  bonds result from the head-to-head overlap of atomic orbitals.
C) A triple bond is composed of two  bonds and one  bond.
D)  bonds have electron density on the internuclear axis.
E) More than one of these statements are correct.
2. How many of the following molecules have all of their atoms in the same plane?
C2H4
H2O2 H2CO SO2 PF3 C5H4 N2H4
A) 4
B) 6
C) 5
D) 7
E) None of the above
3. A certain solid substance that is very hard, has a high melting point, and is nonconducting
unless melted is most likely to be
A) I2
B) H2S
C) NaCl
D) NO2
E) Cu
4. Which of the following should have the highest surface tension at a given temperature?
A) CI4 (carbon tetraiodide)
B) CF4
C) CBr4
D) CCl4
6
5. Which of the following has resonance structures?
A) XeOF2
B) H2CO
C) I3D) N2H4
E) None of the above have resonance structures.
6. Specify the hybridization of the nitrogen atom in each of the following, in order.
NO3–
N2
NO2–
3
A) sp , sp, sp
B) sp3, sp2, sp3
C) sp2, sp, sp3
D) sp2, sp, sp2
E) None of the above
7