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Empirical = Simplest whole number
ratio in a molecule (NOT actual amount
of atoms in a molecule)
Molecular = The exact amount of atoms
in a molecule. (NOT the simplest whole
number ratio)
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What's the big Deal???
O
O
H
O
H
HO
H
H2O2
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Hydroxide
Hydrogen
Peroxide
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Calculating Empirical Formulas
Th
so is s
aw tuf
es f is
om
e!
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Calculating Empirical Formula
1) Write the formula using the number of
moles as the subscripts for each element.
P.5H1.5
2) Divide by the smallest number of moles
P.5H1.5
.5
.5
=
PH3
Empirical
Formula
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Calculating Empirical Formula
C1.5H2
C1.5H2
=
CH1.33
1.5 1.5
CH1.33 x 3 = C3H4
Multiply both
elements to
get a whole
number
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Calculating Empirical Formula
1) Convert each mass to moles
2) Divide by the smallest number of moles
3) If answer is not a whole number,
multiply the whole formula to get rid of
decimal.
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Analysis of a sample of a sulfur acid shows it to contain 0.17 g of hydrogen. 2.82 g of sulfur, and 5.67 g of oxygen. 10
Analysis of a sample of a sulfur acid shows it to contain 0.17 g of hydrogen. 2.82 g of sulfur, and 5.67 g of oxygen. 11
Analysis of a salt results in the following composition: 3.47 g of sodium; 2.12 g of nitrogen; and 7.27 g of oxygen
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Calculating Empirical Formula
% composition is measured by mass
Step 1 Convert from mass % to grams
(assume 100g sample)
Step 2 Convert from grams to moles
Step 3 Divide by smallest # of moles
Step 4 Make sure that your numbers are
whole numbers
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14.3% nitrogen; 4.1% hydrogen; 81.6% bromine
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14.3% nitrogen; 4.1% hydrogen; 81.6% bromine
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24.7% potassium; 34.7% manganese; 40.5% oxygen
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Calculating Molecular from
Empirical Formula
Calculate the formula mass for the
empirical formula.
Divide the molecular mass by the
empirical mass. (this is your multiplier)
Multiply the whole formula by the
multiplier
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Example:
The empirical formula of a compound
is HO. Its molecular mass is found to
be 51g/mol. What is the molecular
formula?
HO = 17 g/mol
Molecular mass = 51 = 3
Empirical mass
17
Empirical x 3 = H3O3
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The empirical formula of a compound is
C2H3 Experiments have yielded that the
molecular mass of the compound is
162.02 grams/mol. What is the molecular
formula?
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A compound is composed of 63.5% silver; 8.2% nitrogen; 28.3% oxygen. The compound is determined to have a molar mass of 339.74 grams. What is the formula of the compound?
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