Download 4 - Practice Calculations - Empirical formulas and % by mass

Empirical and Molecular Formula Questions
1. Use the information provided to determine the empirical and molecular formula for each of
the following:
a. 41.39% carbon, 3.47% hydrogen, and 55.14% oxygen; experimental molar mass:
116.07g
b. 54.53% carbon, 9.15% hydrogen, and 36.32% oxygen; experimental molar mass: 88g
c. 64.27% carbon, 7.19% hydrogen, and 28.54% oxygen; experimental molar mass
168.19g
d. A hydrocarbon containing 17.4% hydrogen by mass, with a molar mass of 58.1g.
2. Use the following information to determine the empirical and molecular formula for each
of the following:
a. 5.00g of a sample is found to contain 3.10g carbon, 0.695g hydrogen and 1.205g
nitrogen. Its molar mass is 116.2g
b. 2.60g of a sample is found to contain 2.26g carbon. It has a molar mass of 126.3g
c. 0.675 g of a sample is found to contain 0.268g carbon, 0.011g hydrogen and 0.396g
chlorine. It has a molar mass of 181.4g
3. Ninhydrin is a compound that reacts with amino acids and proteins to produce a darkcoloured complex. It is used by forensic chemists and detectives to see fingerprints that
might otherwise be invisible. Ninhydrin’s composition is 60.68% carbon, 3.40% hydrogen,
and 35.92% oxygen. What is the empirical formula for ninhydrin? Ninhydrin’s molecular
mass is 178.14, what is its molecular formula?
4. Histamine is a chemical produced during allergic reactions. People sometimes take
antihistamine drugs to counteract the effects of histamine. A sample of histamine having a
mass of 385 mg is composed of 208 mg of carbon, 31 mg of hydrogen, and 146 mg of
nitrogen. The molar mass of histamine is 111 g/mol. What is the molecular formula for
histamine?
5. 0.974 grams of quinine (molar mass = 324 g/mol) is combusted and found to produce 2.642
g CO2, 0.650 g H2O and 0.0842 g nitrogen. Determine the empirical and molecular formulas.
6. The combustion of 80.2 g of a compound which contains only C, H, Cl and O yields 118 g of
CO2 and 30.0g of H2O. Another sample of the compound with a mass of 150 g is found to
contain 44.1 g of Cl. What is the empirical formula of the compound?
ANSWERS: 1 a) CHO, C4H4O4, b) C2H4O, C4H8O2, c) C3H4O, C9H12O3 ,d)C2H5, C4H10, 2 a) C3H8N, C6H16N2 , b) CH2, C9H18 , c)C2HCl, C6H3Cl3, 3)
C9H6O4, C9H6O4, 4) C5H9N3, C5H9N3, 5)C10H12NO, C20H24N2O2 , 6) C4H5ClO2
Adapted from: http://go.hrw.com/resources/go_sc/mc/HUGPS073.PDF