1
Chem 1000A Midterm 2 – Spring 2005 – Solutions
B
March 11th, 2005: 8:00 to 8.50 am
Your name _______________
Instructor: Dr. M. Gerken
Student ID _______________
Time: 50 min
No. of pages: 3+1
Report all your answers using significant figures. Use SI units and show all units and their conversions
throughout your calculations.
Q1
Q2
Q3
Q4
Total Percent
8.5
5
7.5
8
29
100 %
Question 1 (8.5 Marks)
Balance the following redox reaction in basic aqueous solution and show that the final equation has the
correct electron, material, and charge balances. Specify which half-reaction is the oxidation and which
one the reduction in this redox reaction. Show as many steps as possible.
HXeO4- → XeO64- + Xe
H+IXe+VIO-II4- → Xe+VIIIO-II6 4- + Xe0
oxidation half-reaction (unbalanced): HXeO4- → XeO64reduction half-reaction (unbalanced): HXeO4- → Xe
principle element balance:
oxidation half-reaction (unbalanced): HXeO4- → XeO64reduction half-reaction (unbalanced): HXeO4- → Xe
oxygen balance:
oxidation half-reaction (unbalanced): 2H2O + HXeO4- → XeO64reduction half-reaction (unbalanced): HXeO4- → Xe + 4H2O
hydrogen balance (basic solution – 1st step):
oxidation half-reaction (unbalanced): 2H2O + HXeO4- → XeO64- + 5H+
reduction half-reaction (unbalanced): 7H+ + HXeO4- → Xe + 4H2O
hydrogen balance (basic solution – 2nd step):
oxidation half-reaction (unbalanced): 2H2O + HXeO4- + 5OH- → XeO64- + 5H2O
reduction half-reaction (unbalanced): 7H2O + HXeO4- → Xe + 4H2O + 7 OHInserting electrons:
oxidation half-reaction (balanced): 2H2O + HXe+VIO4- + 5OH- → Xe+VIIIO64- + 5H2O +2e-| ×3
reduction half-reaction (balanced): 6e- + 7H2O + HXe+VIO4- → Xe0 + 4H2O + 7 OHelectron balance:
oxidation half-reaction (balanced): 6H2O + 3HXeO4- + 15OH- → 3XeO64- + 15H2O +6ereduction half-reaction (balanced): 6e- + 7H2O + HXeO4- → Xe + 4H2O + 7 OHcombination: 13H2O +4HXeO4- +15OH-+6e- → 3XeO64- +19H2O +6e- +Xe + 7 OHsimplification: 4HXeO4- +8OH- → 3XeO64- +6H2O +Xe
material balance: 4 Xe, 12 H, 24 O| 4Xe, 24 O, 12 H
charge balance:
12- | 12correct electron, material, and charge balance!
2
Question 2 (5 Marks)
Determine the oxidation states of all atoms in the following compounds:
(a) H-O-F
H+I-O0-F-I
(b) Cr2O72O
O
O
Cr
O Cr
-O O- O (only
one resonance structure is shown, the lone pairs are not shown)
Cr+VI2O-II72-
Question 3 (7.5 Marks)
Calculate the volume of O2(g) (in L) at 20°C and 680. Torr required for the complete combustion of 0.268
g of benzoic acid (C7H6O2). Hint: you need to write the balanced reaction equation.
2 C7H6O2 + 15 O2 → 6 H2O + 14 CO2
pV = nRT; for O2: V = ?; T = (20 + 273.15)K = 293 K;
p = 680 Torr × 101325 Pa/760 Torr = 90700 Pa
M(C7H6O2)=122.123 g/mol
n(C7H6O2) = 0.268 g/(122.123 g/mol) = 2.19 × 10-3 mol
n(O2) = mol O2 = mol C7H6O2 × 15 mol O2/2 mol C7H6O2
=2.19 ×10-3 molC7H6O2 ×15 mol O2/2 mol C7H6O2 = 0.0165 mol O2
V = nRT/p =0.0165 mol× 8.3145 J K-1mol-1 ×293 K/(90700 Pa) = 4.44 ×10-4 J/Pa
= 4.44 ×10-4 kg m2 s-2 kg-1 m s2 =4.44 ×10-4 m3 ×(1L/0.001 m3) = 0.444 L
3
Question 4 (8 Marks)
Krypton and sulfur form highly reactive, unstable difluorides: KrF2 and SF2. (a) Draw their Lewis
structures, (b) determine their electron-pair geometries and molecular geometries according to the VSEPR
model, and (c) determine whether these two compounds are polar or non-polar.
..
.. .. .. ..
:F
.. Kr
.. :
.. F
F
: Kr
:
:
F
five electron-pair domains
electron-pair geometry: trigonal bipyramidal
molecular geometry: linear
KrF2 has no molecular dipole moment
and is non-polar
.. ..
S
: ..
F:
F: :..
..
S
F
F
four electron-pair domains
electron-pair geometry: tetrahedral
molecular geometry:bent
SF2 has a molecular dipole moment
and is polar
4
Prefixes
Pico, p 10-12; nano, n 10-9; micro, µ 10-6; milli, m 10-3; centi, c 10-2; deci, d
Fundamental Constants
Proton mass
Avogadro's number 6.022 × 1023 mol-1
-19
Elementary charge, e 1.6022 × 10 C
Neutron mass
molar volume of gases at STP
Electron mass
9.1095 × 10-28 g
-1
-1
Gas constant, R
8.314 J K mol
m
m
pV = nRT
ρ=
n=
M
V
10-1; kilo, k
103
1.67252 × 10-24 g
1.6749 × 10-24 g
22.414 L/mol
Physical quantity
Unit
Symbol
Definition
Frequency, f or ν
Energy , W or E
Force, F
Pressure, p
hertz
joule
newton
pascal
Hz
J
N
Pa
s-1
kg m2 s-2
J m-1 = kg m s-2
N m-2 = kg m-1 s-2
Temperature: 0 K = -273.15 °C; 0 °C = 273.15 K
Pressure: 1 atm = 760 Torr = 760 mmHg = 1.01325 bar = 101325 Pa; 1 bar = 105 Pa
Volume: 1 mL = 1 cm3; 1 L = 1000 cm3 = 1 dm3 = 0.001 m3
1
Chem 1000 Standard Periodic Table
Electronegativities in [ ]
1.0079
1H
18
4.0026
2He
hydrogen
[2.1]
6.941
2
13
14
15
16
17
helium
9.0122
10.811
12.011
14.0067
15.9994
18.9984
20.1797
3Li
4Be
5B
6C
7N
8O
9F
10Ne
11Na
12Mg
magnesium
lithium berrylium
[1.0]
[1.5]
22.9898 24.3050
sodium
[1.0]
39.0983
19K
[1.2]
40.078
3
4
5
6
7
8
9
10
11
44.9559
47.88
50.9415
51.9961
54.9380
55.847
58.9332
58.693
63.546
20Ca
21Sc
22Ti
23V
24Cr
25Mn
26Fe
27Co
28Ni
29Cu
potassium calcium scandium titanium vanadium chromium manganese
iron
[1.6]
[0.9]
[1.0]
[1.3]
[1.4]
[1.5]
[1.6]
[1.7]
(98)
85.4678
101.07
87.62
88.9059 91.224 92.9064
95.94
37Rb
38Sr
rubidium strontium
[0.9]
[1.0]
132.905 137.327
55Cs
cesium
[0.8]
(223)
87Fr
francium
[0.8]
39Y
40Zr
41Nb
yttrium zirconium niobium
[1.3]
[1.4]
[1.5]
178.49 180.948
56Ba
La-Lu
88Ra
Ac-Lr
barium
[1.0]
(226.025)
72Hf
42Mo
[1.6]
183.85
104Rf
rutherfordium
74W
75Re
105Db
106Sg
107Bh
144.24
(145)
rhenium
[1.7]
(264)
dubnium seaborgium bohrium
138.906
140.115
140.908
57La
58Ce
59Pr
89Ac
90Th
91Pa
protactinium
92U
nitrogen
[3.0]
30.9738
oxygen
[3.5]
32.066
fluorine
[4.0]
35.4527
39.948
13Al
14Si
15P
phosphorus
16S
chlorine
[3.0]
79.904
18Ar
[2.1]
74.9216
sulfur
[2.5]
78.96
17Cl
33As
34Se
35Br
36Kr
51Sb
52Te
83Bi
65.39
aluminum
[1.5]
69.723
silicon
[1.8]
72.61
30Zn
31Ga
32Ge
germanium
cobalt
[1.7]
102.906
nickel
[1.8]
106.42
copper
[1.8]
107.868
zinc
[1.6]
112.411
gallium
[1.7]
114.82
45Rh
46Pd
47Ag
48Cd
49In
76Os
78Pt
79Au
77Ir
osmium
[1.9]
(269)
iridium
[1.9]
(268)
108Hs
109Mt
150.36
151.965
hassium meitnerium
platinum
[1.8]
(271)
93Np
80Hg
[1.9]
118.710
50Sn
tin
[1.8]
207.19
arsenic selenium bromine
[2.1]
[2.4]
[2.8]
121.757 127.60 126.905
antimony tellurium
[1.9]
[2.1]
208.980
(210)
53I
82Pb
158.925
162.50
164.930
167.26
168.934
173.04
174.967
66Dy
67Ho
dysprosium holmium
68Er
69Tm
70Yb
71Lu
(244)
(243)
(247)
94Pu
95Am
96Cm
97Bk
uranium neptunium plutonium americium curium
(251)
(252)
erbium
(257)
54Xe
86Rn
81Tl
terbium
(247)
krypton
[2.9]
131.29
85At
thallium
[1.6]
65Tb
83.80
84Po
mercury
[1.7]
bismuth polonium astatine
[1.8]
[1.9]
[2.1]
110Ds
157.25
argon
xenon
[2.3]
(222)
gold
[1.9]
lead
[1.7]
neon
iodine
[2.5]
(210)
darmstadtium
60Nd
61Pm
62Sm
63Eu
64Gd
praesodymium neodymium promethium samarium europium gadolinium
lanthanum cerium
(227.028) (232.038) (231.036) (238.029) (237.048)
actinium9 thorium
44Ru
carbon
[2.5]
28.0855
ruthenium rhodium palladium silver cadmium indium
[1.6]
[1.7]
[1.7]
[1.8]
[1.8]
[1.6]
[1.6]
186.207
190.2
192.22
195.08 196.967 200.59 204.383
73Ta
hafnnium tantalum tungsten
[1.3]
[1.4]
[1.5]
(262)
(266)
(261)
radium
[1.0]
43Tc
molybdenum technetium
12
boron
[2.0]
26.9815
thulium ytterbium lutetium
(258)
(262)
(259)
98Cf
99Es
100Fm 101Md
102No
103Lr
berkelium californium einsteinium fermium mendelevium nobelium lawrencium
radon