Empirical and Molecular Formulas
1. Determine the mass percent composition of Na2CO3
45.980
12.011
(100) = 43.382% Na
(100) = 11.332% C
105.988
105.988
47.997
(100) = 45.285% O
105.988
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
31.6
= 1.32 mol Cr
= 1.98 mol O
51.996
15.999
1.98 / 1.32 = 1.5
CrO1.5  Cr2O3
3. Determine the molecular formula of a compound with an empirical formula
of NH2 and a molar mass of 32.06 g / mol.
14.007 + 2 (1.0079) = 16.023
32.06 / 16.023 = 2
N 2 H4
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
17.83
3.74
= 0.1855 Pb
= 1.484 C
= 3.71 H
207.2
12.011
1.0079
1
1.484 / 0.1855 = 8
3.71 / 0.1855 = 20
PbC8H20
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
8.20
28.3
= 0.589 Ag
= 0.583 N
= 1.77 O
107.87
14.007
15.999
1
1
1.77 / 0.583 = 3
AgNO3
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
67.49
72.67
= 1.298 Na
= 1.298 Cr
= 4.542 O
22.990
51.996
15.999
1
1
4.542 / 1.298 = 3.5
Na2Cr2O7
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 = O3C3N3Cl3
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
94.0735
= 5.8800 H
= 5.8799 O
1.0079
15.999
Close enough HO (empirical)
34.01468
H2O2 (molecular)
= 2 so
1.0079 +15.999
10. Determine the percent composition of CuBr2
CuBr2
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
C6H6
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
8.70
17.3
= 6.16 C
= 8.63 H
= 1.24 N
12.011
1.0079
14.007
6.16
8.63
So C5H7N
Empirical formula
=5C
=7 H
1.24
1.24
162.1 / 81.117 =
so C10H14N2 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
2.77956
13.4050
44.0697
= 3.3091
= 2.7578
= 0.55153
= 0.5515
12.011
1.0079
24.305
79.904
3.3091
2.7578
Empirical formula
=6
= 5 so C6H5MgBr
0.55153
0.55153
181.313 / 181.315 = 1
C6H5MgBr Molecular Formula