Moles WS 4
Determining Molecular Formulas
The subscripts of a molecular compound show the actual number of atoms of each element in one molecule of
the compound. Unlike an empirical formula that shows the smallest whole number ratio, a molecular formula
shows the actual number of atoms.
e.g. The empirical formula of hydrogen peroxide is HO but the molecular formula is H2O2. The empirical
formula of butane is C2H5 but the molecular formula is C4H10.
Steps to determine a molecular formula:
1. Find the empirical formula.
2. Calculate the ratio of the molecular molar mass to the molar mass of the empirical formula. Multiply the
subscripts in the empirical formula by this number to determine the subscripts in the molecular formula.
A sample of phosphorous and oxygen contains 56% oxygen. If the molecular molar mass is 283.9 g,
what is the molecular formula of the compound?
1. Find the number of moles of each element (mass  mol conversion).
43.64 g P
x 1 mole P
30.97 g p
= 1.409 mole P
56.36 g O x 1 mole O
16.00 g O
= 3.522 mole O
2. a) Find the mole ratio by dividing each of the moles by the smallest number of moles.
For P: 1.409 mol = 1.00
1.409 mol
For O: 3.522 mol = 2.50
1.409 mol
 Mole ratio: 1 P: 2.5 O
b) Find the lowest whole number ratio by multiplying the mole ratio by 2.
 2 P: 5 O
3. Write the empirical formula: P2O5
4. Find the ratio of the molecular mass to the empirical formula mass.
Multiply the subscripts in the empirical formula by this ratio.
Empirical molar mass P2O5 =141.94 g/mol
Molecular molar mass = 283.9 g/mol
Ratio: 283.9 g = ~ 2
141.94 g

2(P2O5 )= P4O10
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Moles WS 4
Practice Problems
Show your work.
1.
A compound contains 30.45% nitrogen and 69.55% oxygen. If its molecular mass is 92.02
g/mol, what are its empirical and molecular formulae?
2.
A compound contains 42.56 g of palladium and 0.80 grams of hydrogen. The molecular molar
mass was found to be 216.8 g. What are the empirical and molecular formulae of the
compound?
3.
Octane, a compound of hydrogen and carbon found in gasoline has a molecular molar mass of
114.25 g. If the mass percent of hydrogen in octane is 15.75 %, what are the empirical and
molecular formulae?
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Moles WS 4
More Practice Problems
Find the empirical and molecular formulas of a compound composed of…..
1. 10.87 g Fe and 4.66 g O. Its molar mass is 319.2 g/mole.
Empirical Formula_________
Molecular Formula_________
2. 4.20 g N and 12.0 g O. Its molar mass is 324 g/mole.
Empirical Formula_________
Molecular Formula________
3. 4.80 g C and .40 g H. Its molar mass is 78 g/mole.
Empirical Formula_________
Molecular Formula_________
4. 24.1 g C and 5.1 g H. Its molar mass is 58.1 g/mole.
Empirical Formula_________
Molecular Formula________
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