Chemistry 121 (Tyvoll)
KEY FOR EXAMINATION I
June 11, 2007
Part I. Multiple Choice (3 points each -- Total points = 45.)
1. Which of the following is a chemical property of iron?
1. iron melts at 1535°C
2. iron conducts heat
3. iron can be bent into shapes
4. iron rusts on exposure to water and oxygen
5. iron conducts electricity
Only #4 involves a chemical change.
2. A sample has a mass of 297 g and a volume of 33 cm3. What is the identity of the sample?
1. mercury (density = 13.6 g/cm3)
2. copper (density = 8.92 g/cm3)
3. silver (density = 10.5 g/cm3)
4. aluminum (density = 2.7 g/cm3)
5. iron (density = 7.86 g/cm3)
Identity is tied to the density: 297 g / 33 cm3 = 9.0 g/cm3, closest to density of Cu.
3. A 1 cup (31.0 g) serving of Special K cereal contains 60 mg of potassium. What is the
percent by mass of potassium in this cereal?
1. 1.93 %
2. 19.3 %
3. 0.193 %
4. 5.17 %
5. 0.517 %
% = ((0.060 mg)(10-3 g / mg) / 31.0 g) x 100 = 0.193 %
4. A reaction consumes 5.0 g of A and 6.0 g of B. How many grams of C and D should be
obtained if 1A + 3B 2C + 4D?
1. 23 g
2. 11 g
3. 1 g
4. 10 g
From Conservation of Mass, total mass (of products) must also equal 5.0 g + 6.0 g = 11 g
5. Which metric prefix means 1 × 10-9?
1. kilo
2. nano
3. pico
Definition.
4. micro
5. milli
6. How many phosphorus atoms are there in 225 molecules of white phosphorus, P4?
1. 225
2. 900
3. 400
4. 180
5. 450
P atoms = 225 molecules P x 4 atoms P/ molecule P = 900 atoms P
7. Three elements that are likely to have similar chemical and physical properties are
1. Sr, Se, Sb
2. K, Ca, Sc
3. Sc, Ti, V
4. Ca, Sr, Ba
5. H, Fe, F
Ca, Sr, Ba are all in Group IIA – elements in the same group (family) have similar
properties
8. Perform the indicated operations and give the answer, in cm, with the proper number of
significant figures: 12.64 cm − 48 mm + 0.246 m = ? (Be careful with the units!)
1. 32.44 cm
2. 60.886 cm
3. 42.0 cm
4. 32.4 cm
5. 42 cm
12.64 cm − 4.8 cm + 24.6 cm = 32.44 cm (round to 1 decimal place – least precise values)
9. How many significant figures are there in the quantity, 0.01900 g?
1. 1
2. 2
3. 3
4. 4
5. 5
0.01900 starting with the 1 and including all following zeros
10. Which one of the following pairs of substances illustrates the Law of Multiple Proportions?
1. O3, O2
2. D2O, H2O
3. SiO2, Sb2O3
4. KCl, MgCl2
5. PCl3, PCl5
Only pair with the same two elements in different atomic ratios.
11. An 56Fe2+ particle contains
1.
2.
3.
4.
5.
28 protons, 28 neutrons and 26 electrons.
26 protons, 30 neutrons and 24 electrons.
26 protons, 26 neutrons and 26 electrons.
58 protons, 58 neutrons and 56 electrons.
54 protons, 56 neutrons and 52 electrons.
56-26 = 30 neutrons; 26 – 2 = 24 electrons
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12. Naturally-occurring chlorine consists of the two isotopes: chlorine-35 and chlorine-37. The
atomic mass for chlorine is approximately 35.5. What are reasonable estimates of the
relative percentages of chlorine-35 and chlorine-37, respectively?
1. 50, 50
2. 25, 75
3. 75, 25
4. 33, 67
5. 67, 33
35.5 is fairly close to mass of chlorine-35 so largest % chlorine-35 is most likely here
13. Which pair of symbols below represents atoms that are isotopes?
1. 16O,16N
2. 16O,17O
3. H, He
4. He, He+
5. 13C,14N
Isotopes have same elemental symbol, different mass numbers
14. How many electrons are in an atom of
1. 77
2. 76
3. 33
77
Se?
4. 34
5. 16
(see atomic # from periodic table)
15. The average mass of a single mercury (Hg) atom is 3.331 x 10-22 g. Determine the number of
Hg atoms of in a single drop of liquid mercury if the drop has a volume of 0.050 cm3 and
the density of Hg is13.55 g/cm3.
1. 2.17 x 1026 atoms
2. 1.67 x 1021 atoms
3. 1.50 x 1020 atoms
4. 2.03 x 1021 atoms
5. 5.42 x 1020 atoms
# Hg atoms = (0.050 cm3)(13.55 g / cm3)(1 Hg atom/3.331 x 10-22 g) = 2.03 x 1021 atoms
Part II - Total points = 55.
1. (a) (5 points) Give the correct chemical formula for each of the following compounds:
(1) calcium hydroxide
______Ca(OH)2_________________
(2) xenon difluoride
______XeF2____________________
(3) sulfur trioxide
______SO3_____________________
(4) potassium sulfate
______K2SO4___________________
(5) copper(II) bromide
______CuBr2___________________
(b) (5 points) Give the correct chemical name for each of the following compounds:
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(1) Na2CO3
____sodium carbonate ___________
(2) Ni(NO3)2
____nickel(II) nitrate ____________
(3) HClO3
____chloric acid________________
(4) P2S3
____diphosphorus trisulfide _______
(5) SF6
____sulfur hexafluoride __________
2. (4 points) Factor-Label. (a) Old Dan had a blue 1991 Ford Explorer with a gas tank that had
a volume of 79.4 L. If one U.S. gallon = 231 in 3, how many gallons (gal) would this gas tank
hold? Show work for credit!
? gallons = (79.4 L)(1 gal/3.785 L) = 20.98 L
? gallons = 21.0 L (3 significant figures)
(b) (6 points) If Dan’s old Explorer only got 17.0 miles/gallon, calculate how many km could
Dan expect to travel on a full tank of gasoline and then answer this question: With a full
tank, could he expect to make it from Quezaltenango, Guatemala to Minititlan, Mexico, a
dangerous trip, with a total distance of 663 km ? Show work for credit!
? km = (21.0 gal)(17.0 miles/gal)(1.609 km/mile) = 573.8 km
? km = 574 km so old Dan won’t make it to Minititlan to meet his buddies.
3. (10 points) As discussed in class, a person’s body mass index (BMI) is based upon a person’s
mass (in kg) and height. The BMI is defined as the body mass divided by the square of the
height, in meters, or BMI = kg/m2. Usually, if BMI > 25, the person is considered to be
overweight. What is the maximum weight, in pounds (lb), that a person 5 ft 10.5 in tall can
maintain and still have a BMI that does not exceed 25.0? You must show work for credit!
Height: inches = (5 ft)(12 inches/ft) + 10.5 inches = 70.5 inches
m = (70.5 in)(1 m/39.37 in) = 1.7907 m
m2 = (1.7907 m)2 = 3.207 m2
Let BMI = 25 = kg/m2 so,
25.0 = kg/3.207 m2
kg = (25.0 kg/m2)(3.207 m2) = 80.175 kg
? lb = (80.175 kg)(2.205 lb/kg)
? lb = 176.7 = 177 lb
4. (a) (4 points) Naturally-occurring silicon consists of the following isotopes:
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Isotope
silicon-28
silicon-29
silicon-30
Atomic Mass, u
27.97693
28.97649
29.97376
Percent Abundance
92.21
4.70
3.09
Calculate the weighted average atomic mass of silicon using these data.
Avg. atomic mass = (0.9221)(27.97693) + (0.0470)(28.97649) + (0.0309)(29.97376)
Avg. atomic mass = 25.797 + 1.361 + 0.9261 = 28.085 = 28.08 u (to 2 decimal places)
(b) (6 points) Calculate the percent natural abundances for rubidium-85 (84.91179 u) and for
rubidium-87 (86.90919 u) if they are the only two naturally-occurring isotopes of Rb. Hint:
let “x’ represent the fractional abundance of one of the isotopes and remember that you have
a periodic table available. Show all work for credit!
From periodic table, we find that avg. atomic mass for Rb is 85.47 u, so let x = fraction
Rb-85 and 1.00 – x = fraction Rb-87:
Then, 85.47 = 84.91179 x + (1.000 – x) (86.90919
Solving algebraically for x: x = 0.7205 (72.05%) for Rb-85
and 100.00 - 72.05 = 27.95% for Rb-87
5. (10 points) You have a roll of aluminum wire that has a mass of 225 kg. The wire is to be
used to make finishing nails that are 5.00 cm in length and 0.300 cm in diameter (essentially a
cylinder of metal). If the volume of a cylinder is given by V = πr2l, where r is the radius and l
is the length, how many nails can you make from one roll of aluminum wire? The density of
aluminum is 2.698 g/cm3. Show all work for credit!
r = radius = ½ (diameter) = ½ (0.300 cm) = 0.150 cm
First, find the volume of 1 nail: V = πr2l = π(0.150 cm)2(5.00 cm) = 0.353 cm3
Next, find the mass of 1 nail: g = (0.353 cm3)(2.698 g/cm3) = 0.954 g
? nails = (225 kg)(103 g/kg)((1 nail/0.954 g)
? nails = 2.36 x 105 = 236,000 nails
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6. (5 points) Identify the name of the elements represented by the following group and period
numbers and state whether the element is a metal, a nonmetal or a metalloid.
Element Name
Metal/nonmetal/metalloid
(a) group 4A, period 3
_silicon ______
___metalloid__________
(b) group 7B, period 4
_manganese___
___metal_____________
(c) group 3A, period 4
_gallium______
___metal_____________
(d) group 2B, period 3
_Doesn’t exist_
___Doesn’t exist_______
(e) group 7A, period 5
_iodine_______
___nonmetal __________
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