Molecular Formula

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Molecular Formula
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Percent Composition

• The percent by mass of each element in a compound

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To find percent composition, take molar mass of
element divided by molar mass of compound and
multiply by 100.
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What is the percent composition of H in water.
(2.02 g H 18.02 g water) X 100 11.2 H
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What is the percent composition of O in Glucose
(C6H12O6).
(96 g O 180.18 g Glucose) X 100 53.28 O
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Empirical Formula

• The formula of a compound with the smallest whole
  number mole ration of the elements.

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To find empirical formula, take the grams of each
element and convert to moles by dividing each
element by their molar mass. Then take each mole
and divided by the smallest mole. This gives you
the subscript for each empirical formula.
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What is the empirical formula with 40.05g S and
59.95g O.
40.05g S 32.07g/mol S 1.249 mol
S 59.95gO16g/mol O3.747mol O 1.249 mol S/1.249
1 mol S 3.747 mol O/1.249 3 mol O SO3
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What is the empirical formula with 35.98g Al and
64.02g S.
64.02g S32.07g/molS1.99molS 35.98gAl26.98g/molA
l1.33molAl 1.33 mol Al/1.33 1 mol Al 1.99 mol
S/1.33 1.5 mol S Al2S3
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Molecular Formula

• It specifies the actual number of atoms of each
  element in one molecule or formula unit of the
  substance.

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To find the molecular formula, take the
experimental mass and divided by the mass of the
empirical formula. Take this ratio and multiply
by empirical formula to find molecular formula.
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What is the molecular formula of a compound with
empirical formula of CH and exp. mass of 26.04
g/mol.
26.04 g/mol 13.02 g/mol 2 C2H2
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What is the molecular formula of a compound with
empirical formula of CH and exp. mass of 78.12
g/mol.
78.12 g/mol 13.02 g/mol 6 C6H6