Practice Test #2:
Molar Mass, Percent
Composition, Moles,
Hydrates, and Empirical
and Molecular Formulas
Answers:
1.
2.
3.
4.
5.
c
a
b
d
b
6. a
7. d
8. c
9. a
10. d
1. Theobromine is a compound found in cocoa, the main
ingredient in chocolate. Its formula is C7H8N2O3. What
is the percentage of nitrogen found in theobromine?
C 7·12.0 = 84.0
H 8·1.01 = 8.08
N 2·14.0 = 28.0
O 3·16.0 = 48.0
168.08 g/mol
28.0
·100 = 16.66% N
168.08
2. If a sample of theobromine contains 2.27 x 1019
carbon atoms, how much will the sample weigh?
168 g
1 mole
C7H8N2O3 C7H8N2O3
1 mole
6.022 x 1023
molecules
C7H8N2O3
C7H8N2O3
1
molecule
C7H8N2O3
2.27 x 1019
C atoms
7C
atoms
= 9.05 x 10-4 g C7H8N2O3
3. Chemical Compound Z contains 29.98% carbon, 6.73% hydrogen, 23.32%
nitrogen, and 39.97% oxygen. The molar mass of compound Z is 480. g/mol.
What are the empirical and molecular formulas of compound Z?
1 mol C
29.98 g C
12.01 g C
1 mol H
1.01 g H
= 2.496 mol C
= 1.5 C ·2 = 3 C
1.665 mol
6.73 g H = 6.66 mol H
1.665 mol
1 mol N
23.32 g N
14.01 g N
= 1.665 mol N
1.665 mol
= 4 H ·2 = 8 H
= 1 N ·2 = 2 N
1 mol O 39.97 g O = 2.498 mol O = 1.5 O ·2 = 3 O
1.665 mol
16.00 g O
Empirical formula is C3H8N2O3
MM = 120. g/mol
480. g/mol = 4
120. g/mol
Molecular formula is C12H32N8O12
4.
Laboratory
experiments were
conducted to
determine the
amount of water in
samples of a
hydrate of
magnesium sulfate.
The hydrate was
heated gently. The
anhydrous salt
remained after the
water was
evaporated from
the samle. The
percentage of
water in the
sampleis 51.14%.
What is the
formula for the
hydrate?
magnesium sulfate, 51.14% MgSO4 ⋅ x H2O
Mg
1 ⋅ 24.3
= 24.3
S
1 ⋅ 32.1
= 32.1
O
4 ⋅ 16.0
= 64.0
H2O
x ⋅ 18.0
= 18.0x
(120.4 + 18x) g/mol
18.0x
⋅ 100 = 51.14%
(120.4+ 18x)
18x
(120.4+ 18x)
= 0.5114
18x = 0.5114(120.4+ 18x)
18x = 61.57 + 9.2052x
8.7948x = 61.57
x = 7 ... MgSO4 ⋅ 7 H2O
magnesium sulfate heptahydrate
5. What is the molar mass of phosphoric acid?
H3PO4
H 3·1.01 = 3.03
P 1·31.0 = 31.0
O 4·16.0 = 64.0
98.03 g/mol
6. What is the percentage of hydrogen in phosphoric acid?
H3PO4
H 3·1.01 = 3.03
P 1·31.0 = 31.0
O 4·16.0 = 64.0
98.03 g/mol
3.03
98.03
·100 = 3.09% H
7.
How many hydrogen atoms are in a sample of
phosphoric acid that weighs 11.1g?
3H
atoms
1
molecule
H3PO4
6.022 x 1023
molecules
H3PO4
1 mole
H3PO4
1 mole
98.0 g
H3PO4
H3PO4
= 2.05 x
23
10
11.1 g
H3PO4
H atoms
8. Octadiene contains 88.80% carbon and 11.20% hydrogen. If the molar mass
of octadiene is 162 g/mol, what are the empirical and molecular fomulas?
1 mol C
88.80 g C
12.01 g C
= 7.394 mol C
7.394 mol
= 1 C ·2 = 2 C
1 mol H 11.20 g H = 11.11 mol H = 1.5 H ·2 = 3 H
7.394 mol
1.008 g H
Empirical formula is C2H3
MM = 27. g/mol
162 g/mol = 6
27.0 g/mol
Molecular formula is C12H18
9. What is the mass of a sample of technetium (VII)
oxalate which contains 3.85 x 1040 oxygen atoms?
812 g
1 mole
Tc2(C2O4)7 Tc2(C2O4)7
1 mole
6.022 x 1023
molecules
Tc2(C2O4)7
Tc2(C2O4)7
1
molecule
Tc2(C2O4)7
3.85 x 1040
O atoms
28 O
atoms
= 1.85 x 1018 g Tc2(C2O4)7
10.
Alunogen
is a hydrate
of
aluminum
sulfate that
contains
48.63%
water.
What is the
coefficient
of water in
the
hydrate?
aluminum sulfate, 48.63% Al2(SO4)3 ⋅ x H2O
Al
2 ⋅ 27.0
= 54.0
S
3 ⋅ 32.1
= 96.3
O
12 ⋅ 16.0
= 192
H2O
x ⋅ 18.0
= 18.0x
(342.3 + 18x) g/mol
18.0x
⋅ 100 = 48.63%
(342.3+ 18x)
18x
(342.3+ 18x)
= 0.4863
18x = 0.4863(342.3+ 18x)
18x = 166.46 + 8.7534x
9.2466x = 166.46
x = 18 ... Al2(SO4)3 ⋅ 18 H2O