Answers for Practice Test #1:
Molar Mass, Percent Composition,
Hydrates, Moles, and Empirical and
Molecular Formulas
Answers
1. a
2. d
3. a
4. a
5. d
6. d
7. b
8. c
9. c
10. c
1. What is the molar mass of cryolite,
Na3AlF6? Round to four significant digits.
Na 3·22.99 = 68.97
Al 1·26.98 = 26.98
F
6·19.00 = 114
209.95 g/mol ≈ 210.0 g/mol
2. Nitric acid contains what percent hydrogen by mass?
HNO3
H 1·1.01 = 1.01
N 1·14.0 = 14.0
O 3·16.0 = 48.0
63.01 g/mol
1.01 ·100 = 1.60% H
63.01
3. Adipic acid contains 49.32% carbon, 6.85 % hydrogen and
43.84% oxygen by mass. What is the empirical formula?
1 mol C 49.32 g C = 4.107 mol C
= 1.5 C ·2 = 3 C
12.01 g C
2.740mol
1 mol H
1.01 g H
6.85 g H = 6.78 mol H
2.740 mol
1 mol O 43.84 g O
16.00 g O
= 2.740 mol O
2.740 mol
= 2.5 H ·2 = 5 H
= 1 O ·2 = 2 O
Empirical formula is C3H5O2
4. Vivianite is a hydrate of iron (II) phosphate that contains
28.72% water. What is the coefficient of water in the hydrate?
Fe3(PO4)2  x H2O
18.0x
·100 = 28.72%
357.4+ 18.0x
Fe
3·55.8 = 167.4
18.0x
357.4+ 18.0x
P
2·31.0 = 62.0
18.0x = 0.2872(357.4 + 18.0x)
O
8·16.0 = 128
18.0x = 102.6 + 5.170x
H2O x·18.0 = 18.0x
357.4 + 18.0x g/mol
= 0.2872
12.83x = 102.6
x=8
Fe3(PO4)2 8 H2O
5. What is the molar mass of fluorapatite, Ca5(PO4)3F?
Ca
5·40.1 =
200.5
P
3·31.0 =
93.0
O
12·16.0 = 192
F
1·19.0 =
19.0
504.5 g/mol
6. Vitamin C contains 40.90% carbon, 4.58% hydrogen, and 54.52%
oxygen. The molecular mass of vitamin C is 180. g/mol. The
molecular formula for vitamin C is:
1 mol C 40.90 g C = 3.405 mol C
12.01 g C
3.408 mol
1 mol H
1.01 g H
4.58g H = 4.53 mol H
3.408 mol
1 mol O 54.52 g O
16.00 g O
= 3.408 mol O
3.408 mol
= 1 C ·3 = 3 C
= 1.3 H ·3 = 4 H
= 1 O ·3 = 3 O
Empirical formula is C3H4O3 MM = 88.0 g/mol
180. g/mol = 2
88.0 g/mol
Molecular formula is C6H8O6
7. The empirical formula of styrene is CH. Its molecular mass is
104.1 g/mol. What is the molecular formula of styrene?
Empirical formula is CH
104.1 g/mol = 8
13.0 g/mol
Molecular formula is C8H8
MM = 13.0 g/mol
8. Roundup, a herbicide manufactured by Monsanto, has the formula
C3H8NO5P. How many hydrogen atoms are in 74.2 grams of roundup?
8H
atoms
6.022 x 1023
molecules
C3H8NO5P
1 mole
74.2 g
C3H8NO5P C3H8NO5P
1 molecule 1 mole
169 g
C3H8NO5P C3H8NO5P C3H8NO5P
= 2.12 x 1024 H atoms
9. Iodous acid contains what percent oxygen by mass?
HIO2
H 1·1.01 = 1.01
I
1·127 = 127
O 2·16.0 = 32.0
160.01 g/mol
32.0 ·100 = 20.00% O
160.01
10. How many oxygen atoms are in 23.8 grams of iron (III) oxalate?
12 “O”
atoms
6.022 x 1023
molecules
Fe2(C2O4)3
1 mole
23.8 g
Fe2(C2O4)3 Fe2(C2O4)3
1 molecule 1 mole
376 g
Fe2(C2O4)3 Fe2(C2O4)3 Fe2(C2O4)3
= 4.57 x 1023 O atoms