Answers: 1. Na
2
SO
3
6. H
3
PO
4
2. Fe
2
S
3
7. CH
4
O (actually, CH
3
OH, which is methanol)
3. K
2
CrO
4
8. CH
2
O
4. N
2
O
5
9. C
6
H
12
O
6
5. N
2
O
4
10. C
2
H
4
KEY
Chemistry: Percentage Composition and Empirical & Molecular Formula
Solve the following problems. Show your work, and always include units where needed.
1. A compound is found to contain 36.5% Na, 25.4% S, and 38.1% O. Find its empirical formula.
Na
2
SO
3
2. Find the empirical formula of a compound that is 53.7% iron and 46.3% sulfur.
Fe
2
S
3
3. Analysis of a sample of a compound indicates that is has 1.04 g K, 0.70 g Cr, and 0.86 g O. What is
its
empirical formula?
K
2
CrO
4
4. If 4.04 g of nitrogen combine with 11.46 g of oxygen to produce a compound with a molar mass of
108.0g,
what is the molecular formula of this compound?
O 3 mol 0.79O mol 38.2
O g 16
O mol 1
O g 38.1
S 1 mol 0.79S mol 79.0
S g 32
S mol 1
S g 25.4
Na 2 mol 0.79Na mol 59.1
Na g 23
Na mol 1
Na g 36.5
→÷=
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→÷=
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→÷=
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⎟
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⎛
S 32 S 1.51 mol 0.959S mol 45.1
S g 32
S mol 1
S g 46.3
Fe 2 2 Fe 1 mol 0.959Fe mol 959.0
Fe g 56
Fe mol 1
Fe g 53.7
⇒×→÷=
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⎟
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⎜
⎝
⎛
⇒×→÷=
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⎟
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⎞
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⎜
⎝
⎛
O 4 mol 0.0135O mol 054.0
O g 16
O mol 1
O g 0.86
Cr 1 mol 0.0135Cr mol 0135.0
S g 52
Cr mol 1
Cr g 0.70
K 2 mol 0.0135K mol 027.0
K g 39
K mol 1
K g 1.04
→÷=
⎟
⎟
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⎛
→÷=
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→÷=
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⎟
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N
2
O
5
!
KEY – Page 2
Chemistry: Percentage Composition and Empirical & Molecular Formula
5. The molar mass of a compound is 92 g. Analysis of the sample indicates that it contains 0.606 g N
and
1.390 g O. Find the compound’s molecular formula.
NO
2
! !
6. An acid commonly used in the automotive industry is shown to be 31.6% phosphorous, 3.1%
hydrogen,
and 63.5% oxygen. Determine the empirical formula of this acid.
H
3
PO
4
(phosphoric acid)
7. A solvent is found to be 50.0% oxygen, 37.5% carbon, and 12.5% hydrogen. What is the empirical
formula of this solvent.
O 52 O 2.48 mol 0.289O mol 716.0
O g 16
O mol 1
O g 11.46
N 2 2 N 1 mol 0.289N mol 289.0
N g 14
N mol 1
N g 4.04
⇒×→÷=
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⎟
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⇒×→÷=
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⎟
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⎛
g 108 Mass Molecular
g 80 g/mol 16 O 5
g 28 g/mol 14N 2
=
=×
=×
O 2 mol 0.0433O mol 0869.0
O g 16
O mol 1
O g 1.390
N 1 mol 0.433N mol 433.0
N g 14
N mol 1
N g 0.606
→÷=
⎟
⎟
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g 46 Mass Molecular
g 32 g/mol 16 O 2
g 14 g/mol 14N 1
=
=×
=×
( )
formula) (molecular ONNO2
2
g 92g 46
422
⇒∴
CH
3
OH or
CH
4
O (methanol)
8. A particular sugar is determined to have the following composition: 40.0% carbon, 6.7% hydrogen,
and
53.5% oxygen. Determine the empirical formula of this sugar molecule.
CH
2
O (methanol)
KEY – Page 3
Chemistry: Percentage Composition and Empirical & Molecular Formula
9. If the molar mass of the sugar in question #8 is 180.0 g, find the molecular formula of the sugar.
CH
2
O
! !
C
6
H
12
O
6
10. Ethene, a gas used extensively in preparing plastics and other polymers, has a composition of 85.7%
carbon and 14.3% hydrogen. Its molar mass is 28 g. Find the molecular formula for ethane.
CH
2
! !
O 1 mol 3.33O mol 34.3
O g 16
O mol 1
O g 53.5
H 2 mol 3.33H mol 7.6
H g 1
H mol 1
H g 6.7
O 1 mol 3.33C mol 33.3
O g1c
C mol 1
C g 40
→÷=
⎟
⎟
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→÷=
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⎟
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⎛
→÷=
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⎟
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⎞
⎜
⎜
⎝
⎛
g 30 Mass Molecular
g 2 g/mol 1 H 2
g 16 g/mol 16 O 1
g 12 g/mol 12C 1
=
=×
=×
=×
( )
formula) (molecular OHCOCH6
6
g 180g 30
61262
⇒∴
H 2 mol 7.14H mol 3.14
H g 1
H mol 1
H g 14.3
C 1 mol 7.14C mol 14.7
C g 12
C mol 1
C g 85.7
→÷=
⎟
⎟
⎠
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⎜
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⎛
→÷=
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⎟
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⎞
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⎜
⎝
⎛
g 14 Mass Molecular
g 2 g/mol 1 H 2
g 12 g/mol 12C 1
=
=×
=×
( )
formula) (molecular HCCH2
2
g 28g 14
422
⇒∴