CHM 150: Empirical Formulas and Combustion Analysis
1. A 7.546 g sample of an unknown compound was analyzed and found to contain only nitrogen and
oxygen. If the mass of nitrogen present in the sample was 1.957 g, determine the empirical formula
of the compound.
2. Serotonin is a compound that conducts nerve impulses in the brain. Serotonin has the following
percent composition by mass: 68.2% carbon, 6.86% hydrogen, 15.9% nitrogen, and 9.08% oxygen.
a. What is its empirical formula?
b. What is its molecular formula if its molar mass is about 176 g/mol?
3. The koala dines exclusively on eucalyptus leaves. Its digestive system detoxifies the eucalyptus oil
that can poison other animals. The chief constituent in eucalyptus oil is a substance called
eucalyptol, which has the following mass percent composition: 77.87% carbon, 11.76% hydrogen,
and the remainder is oxygen.
a. What is its empirical formula?
b. What is its molecular formula if its molar mass is about 154 g/mol?
4. Determine the molecular formulas for the following compounds given their empirical formulas and
their molar masses: a) isoamyl formate: empirical formula=C3H6O, molar mass=116 g/mol;
b) lactic acid: empirical formula=CH2O, molar mass=90.1 g/mol; and c) opionic acid: empirical
formula=C2H2O, molar mass=210.2 g/mol.
5. Vanillin, the dominant flavor in vanilla, is a hydrocarbon derivative containing carbon, hydrogen,
and oxygen. When 1.050 mg of vanillin is completely combusted, 2.430 mg of carbon dioxide and
0.500 mg of steam are produced. What is the percent composition by mass of each element in
vanillin?
6. Leaded gasoline contains an additive to prevent engine “knocking.” Upon analysis, the additive
compound is found to contain lead (hence, “leaded gasoline”), carbon, hydrogen. When 51.36 g of
this compound are burned, 55.90 g of CO2 and 28.61 g of H2O are produced. What is the percent
composition by mass of each element in leaded gasoline?
7. Because of its detrimental effect on the environment, the lead compound described in Problem 6
above has been replaced in recent years by methyl tert-butyl ether (a hydrocarbon derivative
containing carbon, hydrogen, and oxygen) to enhance the performance of gasoline. (As of 1999, this
compound is also being phased out because of its contamination of drinking water.) When 12.1 g of
the compound are burned, 30.2 g of CO2 and 14.8 g of H2O are formed. What is the percent
composition by mass of each element in the compound?
8. Lysine, an essential amino acid in the human body, contains C, H, O, and N. In one experiment, the
complete combustion of 2.175 g of lysine gave 3.94 g CO2 and 1.89 g H2O. In a separate
experiment, 1.873 g of lysine gave 0.436 g NH3. What is the percent composition by mass of each
element in lysine?
Key
1. N2O5; 2 a. C10H12N2O, b. C10H12N2O; 3 a. C10H18O, b. C10H18O; 4 a. C6H12O2, b. C3H6O3, c. C10H10O5;
5. 63.16% C, 5.33% H, and 31.51% O; 6. 29.70% C, 6.233% H, and 64.07% Pb; 7. 68.1% C, 13.7% H, and
18.2% O; 8. 49.43% C, 9.72% H, 19.1% N, 21.8% O.