Percent Formula
Percent Formula
Percent Formula
Chemical Formulas
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Objectives
Determine the percent composition by
mass of elements in a compound
Differentiate between empirical and
molecular formulas
Calculate the empirical and molecular
formulas given percentages
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Percent Composition
Mass of element x 100
Mass of cmpd
If a 13.60 g sample of magnesium oxide is
decomposed, 5.40 g of oxygen is
obtained.
What is the mass of Mg?
Calculate the percent composition of each
element.
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Percent Composition
Mass of element x 100
Mass of cmpd
If a 13.60 g sample of magnesium oxide is
decomposed, 5.40 g of oxygen is
obtained.
What is the mass of Mg? 8.20 g
Calculate the percent composition of each
element. O: 39.7%; Mg: 60.3%
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If no numbers are given, use the mass in
one mole (molar masses)
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Calculating mass of element
Problem #1: What is the percentage of
carbon in CO2?
27.29% C
Problem #2: How many grams of carbon
are in 25 g of CO2?
6.8 g C
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Chemical Formulas
Molecular formula—the actual, chemical
formula
Empirical formula—the simplified version
of the molec formula
Sometimes molec formulas can’t be simplified
Molec: C6H12O6, C5H10O5
Emp: CH2O
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What molecular formulas can you create
from:
CH
CH2
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Determining Empirical Formulas
A compound contains 75% carbon and 25%
hydrogen. Determine the empirical formula.
1. Turn percentage into grams
75% C → 75 g C
25% H → 25 g H
2. Turn grams into moles
75 g C → 6.2 moles C
25 g H → 25 moles H
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3. Divide by smallest # of moles
6.2 moles C = 1.00
6.2
25 moles H = 4.03
6.2
4. Use whole numbers for empirical formula
CH4
Only round when the number is CLOSE
11
Calculate the empirical formula for a
compound that is 67.6% mercury, 10.8%
sulfur, and 21.6% oxygen.
HgSO4
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Determining Molecular Formulas
Multiply empirical formula by a whole
number to get molecular formula.
Molar mass of molec formula
Molar mass of empirical formula
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A compound has the empirical formula HO
and a molar mass of 34.0 g/mol. What is
the molecular formula?
34.0 g/mol = 2
17.0 g/mol
HO x 2 → H2O2
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Calculate the molecular formula of a
compound whose molar mass is
60.0 g/mol and empirical formula is CH4N.
C 2H 8N 2
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