Ch. 10.4
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Calculate the percent composition of oxygen in
a molecule of water.
Step 1:Find the molar mass of water.
◦ 18.02 g/mol
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Step 2: Use the “Mass Percent” equation to solve.
◦ Mass percent = 16.00 g/mol O
◦
______________ X 100 = 88.79% O
◦
18.02 g/mol H2O
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Calculate the percent composition of all
elements in a molecule of HF (hydrofluoric acid).
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Step 1:Find the molar mass of HF.
◦ 20.00 g/mol
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Step 2: Use the “Mass Percent” equation to solve
for each element.
◦ Mass percent = 1.008 g/mol H
◦
______________ X 100 = 5.040% H
◦
20.00 g/mol HF
◦ Mass percent = 18.99 g/mol F
◦
______________ X 100 = 94.95% F
◦
20.00 g/mol HF
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You can check your work by adding the two
percents together. They should be close to
100%.
◦ 5.040% + 94.95% = 99.99%
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
How many grams of aluminum could be
produced from 30.0 g of AlCl3?
Step 1: Solve for percent mass of aluminum.
◦ 20.24% Al
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Step 2: Multiply this percentage to the original
mass of the compound.
◦ 30.0 g x .2024 = 6.072 g
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Answer: 6.072 g of aluminum could be produced.
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Complete the GREEN Percent Composition
Worksheet in your groups.
You will have 30 min. of class time to work
on this. Whatever you do not finish is
homework.
The answer key is available to check your
work in the answer key folder.
GOOD LUCK!
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
Calculate the empirical formula of a
compound that is 75% carbon and 25%
hydrogen?
Step 1: Assume a 100 g sample of the
compound.
◦ 75% C = 75g C
◦ 25% H = 25g H
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Step 2: Convert from grams of each element
to moles of each element.
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Step 3: Divide both numbers by the smallest
of the two.
◦ 6.2 mol C / 6.2 = 1.0 mol C
◦ 25 mol H / 6.2 = 4.0 mol H
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Step 4: Write the empirical formula.
◦ CH4
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Complete the PINK Determining Empirical
Formulas Worksheet in your groups.
You will have 15 min. of class time to work
on this. Whatever you do not finish is
homework.
The answer key is available to check your
work in the answer key folder.
Notice: The first problem on this sheet has
been done for you!
GOOD LUCK!
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What is the empirical formula for a compound
which contains 5.41g Fe, 4.64g Ti, and 4.65g
O?
◦ Step 1: Convert from grams of each element to
moles of each element.
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Step 2: Divide each number by the smallest of
the three.
◦ .00969 mol Fe/.00969 = 1.00 mol Fe
◦ .00969 mol Ti /.00969 = 1.00 mol Ti
◦ .291 mol O /.00969 = 3.00 mol O
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Step 3: Write the empirical formula.
◦ FeTiO3
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Complete the YELLOW Empirical Formula
Worksheet in your groups.
You will have 15 min. of class time to work
on this. Whatever you do not finish is
homework.
The answer key is available to check your
work in the answer key folder.
GOOD LUCK!
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
Determine the molecular formula of a
compound with an empirical formula of CH
and a molar mass of 78.11 g/mol.
Step 1: Calculate the empirical molar mass.
◦ 1 C= 12.01 g/mol
◦ 1 H= 1.008 g/mol
◦
+ ___________
◦
13.02 g/mol
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Step 2: Calculate “n” using the molar mass
and the empirical molar mass.
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Step 3: Multiply “n” by the empirical formula
to get the molecular formula.
◦ 6(CH) = C6H6
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Notice: In some problems, you may not be
given the empirical formula. In these cases,
you will have to calculate the empirical
formula first, then follow the steps to solve
for molecular formula.