Unit 1 Lecture 7:
Percent Composition, Empirical and
Molecular Formulas
The student can justify the observation that
the ratio of the masses of the constituent
elements in any pure sample of that
compound is always identical on the basis of
the atomic molecular theory.
Mass Percent/ Percent Composition
• Found by dividing the mass of the element
by the mass of the compound and
multiplying by 100.
𝒑𝒂𝒓𝒕𝒆𝒍𝒆𝒎𝒆𝒏𝒕
𝒎𝒂𝒔𝒔 𝒑𝒆𝒓𝒄𝒆𝒏𝒕 =
𝒙 𝟏𝟎𝟎%
𝒘𝒉𝒐𝒍𝒆𝒄𝒐𝒎𝒑𝒐𝒖𝒏𝒅
• Can be determined from molecular formula.
•
NOTE: Subscripts indicate the mole ratio of elements
present in a compound.
•
For iron in iron(III) oxide, (Fe2O3):
(2 𝑚𝑜𝑙 )(
𝑚𝑎𝑠𝑠 % 𝑜𝑓 𝐹𝑒 =
2 𝑚𝑜𝑙
55.85 𝑔
)
𝑚𝑜𝑙
55.85 𝑔
16.00 𝑔
+ 3 𝑚𝑜𝑙 (
)
𝑚𝑜𝑙
𝑚𝑜𝑙
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𝑥 100% = 69.94%
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EXERCISE!
Sodium hydrogen carbonate (NaHCO3), also called
baking soda, is an active ingredient in some antacids
used for the relief of indigestion. Determine the
percent composition each element in NaHCO3 .
EXERCISE!
Consider separate 100.0 gram samples of each of
the following:
H2O, N2O, C3H6O2, CO2
– Rank them from highest to lowest percent
oxygen by mass.
H2O, CO2, C3H6O2, N2O
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Formulas
• A molecular formula of a compound , which
shows the elements symbols and numerical
subscripts, tells you the type and number of
each atom in a molecule.
• The empirical formula is the formula that
shows the smallest whole-number mole ratio
of the compound’s elements.
NOTE: Compound subscripts indicate the mole ratio of elements
present in a particular compound.
• Molecular formula
(actual formula of
the compound)
Benzene
– C6H6 = (CH)6
• Empirical formula
(shows simplest
whole-number
mole ratio)
– CH
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Acetylene
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Calculating Empirical Formula from
Mass Percent
• You can calculate the
empirical formula from
percent by mass by
1. First, assuming you have
100.00 g of the compound.
2. Then, convert the mass of
each element to moles.
3. Finally, divide each mole
value by the smallest value.
• The empirical formula may
or may not be the same as
the molecular formula.
EXERCISE!
Methyl acetate is a solvent commonly used in
some paints, inks, and adhesives. Determine the
empirical formula for methyl acetate, which has
the following chemical analysis: 48.64% carbon,
8.16% hydrogen, and 43.20% oxygen.
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EXERCISE!
Succinic acid is a substance produced by lichens.
Chemical analysis indicates it is composed of
40.68% carbon, 5.08% hydrogen, and 54.24%
oxygen and has a molar mass of 118.1 g/mol.
Determine the empirical and molecular formulas
for succinic acid.
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EXERCISE!
The composition of adipic acid is 49.3% C, 6.9%
H, and 43.8% O (by mass). The molar mass of the
compound is about 146 g/mol.
– What is the empirical formula?
C3H5O2
– What is the molecular formula?
C6H10O4
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EXERCISE!
The mineral ilmenite is usually mined and
processed for titanium, a strong, light, and
flexible metal. A sample of ilmenite contains
5.41 g of iron, 4.64 g of titanium, and 4.65 g of
oxygen. Determine the empirical formula for
ilmenite.
Calculate the simplest ratio by
dividing each mole value by the
smallest value.
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Hydrates
• Hydrates are compounds that incorporate water
molecules in their crystalline structures.
• The ratio of moles of water to one mole of the
compound is a
small whole
number.
– Example: Hydrated
copper(II) sulfate
pentahydrate
(CuSO4·5H2O),
has a
compound:water
ratio of 5:1.
CuSO4·5H2O
Anhydrous forms of hydrates are often used to absorb water, particularly during
shipment of electronic and optical equipment.
In chemistry labs, anhydrous forms of hydrates are used to remove moisture from
the air and keep other substances dry.
Determining Hydrate Formula
• When heated, water molecules are released
from a hydrate leaving an anhydrous
compound.
– Weigh hydrate, then heat to drive
off the water.
– Change in mass = mass of water
lost
– Convert the known masses of H2O
and now anhydrous substance to moles.
– Find ratio of moles of H2O to mole of
anhydrous substance.
EXERCISE!
A mass of 2.50 g of blue, hydrated copper sulfate
(CuSO4•xH2O) is placed in a crucible and heated.
After heating, 1.59 g of white anhydrous copper
sulfate (CuSO4) remains. What is the formula for the
hydrate? Name the hydrate.
1. Weigh hydrate, then heat to drive off the water.
2. Change in mass = mass of water lost
3. Convert the known masses of H2O and now
anhydrous substance to moles.
4. Find ratio of moles of H2O to mole of anhydrous
substance.
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EXERCISE!
A 5.00 g sample of barium chloride hydrate was
heated in a crucible. After the experiment, the
mass of the solid weighed 4.26 g. Determine the
number of moles of water that must be attached
to BaCl2.
1. Weigh hydrate, then heat to drive off the water.
2. Change in mass = mass of water lost
3. Convert the known masses of H2O and now
anhydrous substance to moles.
4. Find ratio of moles of H2O to mole of anhydrous
substance.
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