1.
Determine the mass percent composition of Na
2
CO
3
45.980
105.988
(100)
=
43.382%
Na
12.011
105.988
(100)
=
11.332%
C
47.997
105.988
(100)
=
45.285% O
2.
An oxide of chromium is found to have the following percent composition:
68.4 % Cr and 31.6 % O. Determine the compound’s empirical formula.
68.4
=
1.98
mol O
51.996
=
1.32
mol Cr
31.6
15.999
1.98 / 1.32 = 1.5
CrO
1.5
 Cr
2
O
3
3.
Determine the molecular formula of a compound with an empirical formula of NH
2
and a molar mass of 32.06 g / mol.
14.007 + 2 (1.0079) = 16.023
32.06 / 16.023 = 2 N
2
H
4
4.
A 60.00 g sample of tetraethyl lead, a gasoline additive, is found to contain
38.43 g of lead, 17.83 g of carbon, and 3.74 g of hydrogen. Find the empirical formula.
38.43
1
207.2
=
0.1855 Pb
17.83
=
1.484 C
12.011
1.484 / 0.1855 = 8
PbC
8
H
20
3.74
=
3.71 H
1.0079
3.71 / 0.1855 = 20
5.
The percent composition of a compound was found to be 63.5% silver, 8.20% nitrogen, and 28.3% oxygen. Determine the compound’s empirical formula.
63.5
=
0.589 Ag
107.87
1
8.20
14.007
1
=
0.583 N
28.3
=
1.77 O
15.999
1.77 / 0.583 = 3
AgNO
3
6.
Calculate the percent composition of NaOH
NaOH 57.479% Na 39.978% O 2.5199% H
7.
A 170.00 g sample of an unidentified compound contains 29.84 g of sodium,
67.49 g of chromium, and 72.67 g of oxygen. What is the compound’s empirical formula?
29.84
22.990
1
=
1.298 Na
67.49
51.996
1
=
1.298 Cr
72.67
=
4.542 O
15.999
4.542 / 1.298 = 3.5
Na
2
Cr
2
O
7

8.
The empirical formula for trichloroisocyanuric acid, the active ingredient in many household bleaches, is OCNCl. The molar mass of this compound is
232.41 g/mol. What is the molecular formula for trichloroisocyanuric acid?
15.999 + 12.011 + 14.007 + 35.453 = 77.47 g/mol
232.41 / 77.47 = 3
OCNCl x 3 = O
3
C
3
N
3
Cl
3
9.
A compound containing 5.9265 % H and 94.0735 % O has a molar mass of
34.01468 g/mol. Determine the empirical and molecular formula of this compound.
5.9265
1.0079
=
5.8800 H
94.0735
=
5.8799 O
15.999
Close enough HO (empirical)
34.01468
1.0079
+
15.999
=
2
so H
2
O
2
(molecular)
10.
Determine the percent composition of CuBr
2
CuBr
2
28.451% Cu 71.549% Br
11.
The empirical formula of a hydrocarbon is found to be CH. Laboratory procedures have found that the molar mass of the compound is 78 g/mol.
What is the molecular formula of this compound?
78 / 13.019 = 6 C
6
H
6
12.
The molar mass of nicotine is 162.1 g/mol. It contains 74.0 % carbon, 8.70 % hydrogen, and 17.3 % nitrogen. Determine nicotine’s empirical and molecular formula.
74.0
12.011
=
6.16
C
8.70
1.0079
=
8.63
H
17.3
14.007
=
1.24
N
6.16
=
5
C
8.63
=
7
H
1.24
1.24
162.1 / 81.117 = so C
So C
10
H
5
14
H
N
7
2
N Empirical formula
Molecular Formula
13.
Phenyl magnesium bromide is used as a Gringard reagent in organic synthesis. Determine its empirical and molecular formula if its molar mass is
181.313 g/mol and it contains 39.7458 % C, 2.77956 % H, 13.4050 % Mg, and 44.0697 % Br.
39.7450
12.011
=
3.3091
2.77956
1.0079
=
2.7578
13.4050
24.305
=
0.55153
44.0697
79.904
=
0.5515
3.3091
=
6
2.7578
=
5
so C
6
H
5
MgBr
0.55153
0.55153
181.313 / 181.315 = 1 C
6
H
5
Empirical formula
MgBr Molecular Formula