Chem 107 - Hughbanks
Exam 2, March 12, 2015
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Section 502
Exam 2, Version A
On the last page of this exam, you’ve been given a periodic table and some physical constants. You’ll
probably want to tear that page off to use during the exam – you don’t need to turn it in with the rest of
the exam.
The exam contains 11 problems, with 6 numbered pages. You have 75 minutes to complete the exam.
Please show ALL your work as clearly as possible – this will help us award you partial credit if
appropriate. Even correct answers without supporting work may not receive credit. You may use an
approved calculator for the exam, one without extensive programmable capabilities or the ability to store
alphanumeric information. Print your name above, provide your UIN number, and sign the honor code
statement below.
On my honor as an Aggie, I will neither give nor receive unauthorized assistance on this exam.
SIGNATURE:
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(For grading)
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For questions 1 through 4, consider the following gas samples (assume all the
gases behave ideally):
A. 38 g of F2 (g) at 0 °C in a 22.4 L container
B. 600 torr of N2 (g) at 27 °C in a 100.0 L container
C. 0.50 mol of Ne (g) at 27 °C in a 5.0 L container
READ THESE DIRECTIONS CAREFULLY: For each question below, please
indicate your choice and give a brief justification of your answer (i.e., an
explanation of your reasoning). You shouldn’t need detailed numerical calculations,
although you may perform some if you feel that will help. In the event that more
than one of the gases satisfies one of the statements, you should say so. (For
example, if you think they all are at the same pressure, then for part (b) you would
write “All have the same pressure because....”)
(1) (5 pts) Which gas sample contains the fewest molecules?
Ans. 1
(2)
C
B
(5 pts) In which of the samples is the average kinetic energy of the molecules highest?
Ans. 3
(4)
/5
/5
/5
/5
/12
/4
/7
/20
/10
/12
/15
/100
(5 pts) Which gas sample is at the lowest pressure?
Ans. 2
(3)
1
2
3
4
5
6
7
8
9
10
11
Tot.
B and C
(Same temperature, 300 K, same avg. K.E. and higher that 273 K)
(5 pts) In which of the samples is the average molecular speed slowest?
Ans. 4
A
(Most massive molecules, will be slowest – also lowest avg. K.E. at 273 K)
1
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(5) (12 pts, 3 pts each) Consider an electron in an atomic orbital that has the following quantum
numbers: n = 3, l = 2. (In each case please write your answers on the lines provided.)
(a) (3 pts) What type of orbital (e.g., 1s, 3p, etc.) does this electron occupy?
____3d_____
(b) (3 pts) What is the maximum number of electrons in the same atom that could have these two
quantum numbers?
____10_____
(c) (3 pts) Write the symbol for the lightest element in which an orbital with these quantum numbers
would be occupied in the ground state electronic configuration.
____Sc_____
(d) (3 pts) For the element you listed in (c), how many other electrons would have n = 3 in the ground
state electronic configuration?
___8 (3s23p6)___
(6)
(4 pts) Write out the full electronic configuration of a gallium atom (Ga, Z = 31, no inert gas
formalism).
Ans. 6_______1s22s22p63s23p63d104s24p1_______________
(7)
(7 pts) In a selenium atom, which electron is least effective in screening a 3p electron from the
nucleus,
(a) a 1s electron
(b) a 3s electron
(e) a – d are all equally effective
(c) another 3p electron
(d) a 4p electron
Ans. 7______d______
2
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(8) (20 points) In (a) – (e), please circle your choices. In (f), writing clearly, fill in the blanks with the
choices given. BE SURE TO READ EACH STATEMENT VERY CAREFULLY!
(a) (3 pts) Which one of the following elements is most likely to form a cation with a +2 charge?
Br
Rb
Mg
S
Al
Cl
(b) (3 pts) Which one of the following compounds would be best classified as ionic?
CH3NH2
CO2
GaP
C2H6
CaO
BrCl
(c) (4 pts) Which two of the following atoms or ions would not be paramagnetic in their ground state?
Zn2+
N
Br–
C
Mn2+
B
(d) (3 pts) Which one of the following atoms or ions would have the largest radius?
I
Xe
Ba
Ba+
Rb+
Cs+
(e) (3 pts) Which one of the following atoms or ions would have the smallest ionization energy?
Al3+
F–
Na+
Mg2+
Ne
(f) (4 pts) Put the atoms, molecules, or ions in order, starting with those with the fewest unpaired
electrons to most unpaired electrons: Na, Ni2+, Zn2+, N2, N
Fewest unpaired ___N2___ < ___ Zn2+__ < ___ Na ___ < ___Ni2+___ < ___N____ Most unpaired
3
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Questions 9-11 – show all work (37 pts)
(9) (10 points) When ozone (O3) in the ozone layer absorbs enough energy from light, it can be
hν
dissociated into O2 molecules and O atoms: O3 ⎯⎯
⎯→ O2 + O . With sufficient energy from light,
hν
O2 molecules can be dissociated into two O atoms: O2 ⎯⎯
⎯→ O + O . Both of these processes
require photons in the ultraviolet region of the electromagnetic spectrum (λ < 400 nm).
Considering the relative strength of the bonds in reactants and products in each process, which of
these two processes requires the shortest wavelength radiation to occur? (Clearly justify your
answer with a brief explanation. Hint: draw Lewis structures to help you decide which of the bond
breaking processes requires the most energy.)
O
O
h ν1
O
O
O
O
O
O
+
O
O
O
h ν2
2 O
In the first process, there is the net loss of a single bond. In the second process, a double
bond is broken. The second process requires greater energy and therefore shorter
wavelength.
4
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(10) (12 points) The atmosphere around the sun contains many He+ ions, and the emission of light by
these ions has been studied extensively. If the electron in a He+ ion is excited to the n = 5 energy
level, it can return to the ground state by emitting a series of 3 photons, with wavelengths of 26 nm,
469 nm, and 1014 nm.
(a) (3 pts) Find the energy of the 1014 nm photon.
E=
hc (6.626 × 10−34 J i s)(2.9979 × 108 m i s −1 )
=
λ
(1.014 × 10−6 m)
E = 1.959 × 10−19 J
(b) (3 pts) This process is summarized in the energy level
diagram here, where the 3 arrows represent the transitions
giving rise to the emission wavelengths listed above. (The
diagram is roughly to scale.) Which arrow (A, B, or C)
corresponds to the wavelength whose energy you found in
part (a)? (Briefly, give a reason for your choice.)
The longest wavelength is the lowest energy process, so the
transition labeled ‘A’ was computed in part (a).
(c) (6 pts) What emission wavelength should be observed for emission from the n = 4 level to the
ground state (n = 1)?
E4→1 = E B + EC
hc
hc hc
=
+
λ4→1 λ B λC
1
λ4→1
=
⇒
1
λ4→1
=
1
1
+
λ B λC
1
1
+
= 0.040594 nm −1
469 nm 26 nm
λ4→1 = 24.63 nm
5
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(11) (15 points) Below are shown four neutral molecules and one anion containing carbon-sulfur bonds.
The diagrams given show only the way the atoms are connected together; they are NOT complete
Lewis diagrams. There are five distinct observed C–S bond distances (in pm), in order of
increasing length: 153.5, 155.9, 161, 167, 180.2 pm for the bonds that are labeled below. (Some
bonds have the same length as others, so there are more labeled bonds than there are bond distances
given in this list.) Pay attention to the charges – only CH3CS2– isn’t a neutral molecule.
Fill in the table below. Use Lewis diagrams, including any necessary resonance structures, to find
the C-S bond orders and decide which observed bond distances belong to which molecules. If you
can’t assign an exact value to a bond order, give the best estimate you can by using inequalities (for
example, if you think a bond order is greater than 2 but less than 2.5, write 2 < B.O. < 2.5).
Bond
Bond
order
Bond
distance*
acceptable
alternative
a
3
153.5
153.5
b
2
155.9
161
c
2
155.9
161
d
1
180.2
180.2
e
1
f
2
180.2
161
180.2
155.9
g
1.5
167
167
h
167
167
*Bond distance choices are (in pm):
1.5
153.5, 155.9, 161, 167, 180.2
6
0