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Empirical & Molecular Calculation Practice
Calculating Empirical Formulas
1. What is the empirical formula for a compound that is 82.2% nitrogen and 17.8% hydrogen?
Step 1: Assume 100g sample. Rewrite element percentages as element masses.
Step 2: Convert from mass to moles for each element.
Step3: Identify smallest number of moles. Divide each result from step 2 by this number to establish ratio of elements in
compound.
Step 4: Write compound formula using the results from step 3 as subscripts.
a.
N3H
c.
NH3
b.
NO
d.
N2H6
2. What is the empirical formula for a compound that has 53.3% oxygen and 46.7% silicon?
Step 1:
Step 2:
Step3:
Step 4:
a.
SiO
c.
Si2O
b.
SiO2
d.
Si2O3
3. What is the empirical formula for a compound that is 7.9% Li and 92.1% Br?
a.
LiBr
c.
LiBr3
b.
LiBr2
d.
LiBr4
Calculating Molecular Formulas
1. The chart below gives the empirical formula and molecular mass (the compound’s molar mass) for two
compounds. From the data provided, determine the molecular formula for the compounds.
Empirical Formula
Empirical Formula
Molar Mass (g/mol)
Molecule Molar Mass
(g/mol)
NH2
32
C2HCl
181.5
Molecular Formula
2. Sulfuric acid is one of the most powerful acids in the world. Yet it is used in lead storage batteries like the one
found in your family car and is used in the manufacturing of fertilizer. Given the following information,
determine the Empirical Formula and the Molecular Formula of this compound:
Percent Composition: 32.69% Sulfur, 2.06% Hydrogen, 65.25% Oxygen
Molecular Molar Mass: 98.09 g/mol
3. Hydrazine is a highly toxic and flammable liquid that is used in the formation of polymer foams, pharmaceuticals,
rocket fuels, and to control the amount of dissolved oxygen in conventional and nuclear power plants. Given the
following information, determine the Empirical Formula and the Molecular Formula of this compound:
Percent Composition: 87.40% Nitrogen, 12.60% Hydrogen
Molecular Molar Mass: 32.06 g/mol
4. Ethyne, also known as acetylene, is an organic compound that serves as a raw material for plastics, vinyls, and
chemical fuels. It is also used in welding in the form of acetylene torches. Given the following information,
determine the Empirical Formula and the Molecular Formula of this compound:
Percent Composition: 92.24% Carbon, 7.76% Hydrogen
Molecular Molar Mass: 26.06 g/mol
5. One of the major contributors to Acid Rain is an atmospheric gas composed of nitrogen and oxygen. When
molecules of this gas react with water vapor in the atmosphere, nitric acid becomes suspended in the clouds.
Given the following information, determine the Empirical Formula and the Molecular Formula of this compound:
Percent Composition: 22.59% Nitrogen, 77.41% Oxygen
Molecular Molar Mass: 62.004 g/mol
6. Trace impurities of iron and chromium in natural corundum form the gemstones ruby and sapphire. These gems,
however, are primarily a binary compound of aluminum and oxygen, with 52.9% Al. Given the following
information, determine the Empirical Formula and the Molecular Formula of this aluminum oxide compound:
Percent Composition: 52.93% Aluminum
Molecular Molar Mass: 101.961 g/mol