Percent Composition

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Percent Composition
• Percentage composition of a compound gives
the relative amount of each element present.
• % = mass element x 100
mass compound
• The sum of percentages of each element in a
compound should be = 100
Calculate the percent composition for carbon
dioxide (CO2)
• Calculate the molar mass for each element and the
compound
• C: 1 x 12.0 = 12.0
O: 2 x 16.0 = 32.0
+
44.0 g/mol
• %C = 12 x 100 =27.3 %
44
% O = 32 x 100 = 72.7%
44
• Sum of percentages = 100
• Round percentages to the tenth.
Calculate the percent composition for Al(OH)3
aluminum hydroxide
• Calculate the molar mass for each element and the
compound
• Al: 1 x 27.0 = 27.0
H: 3 x 1.0 = 3.0
O: 3x 16.0 = 48.0
+
78.0 g/mol
• %Al = 27 x 100 = 34.6%
78
% H = 3 x 100 = 3.8%
78
%O = 48 x 100 = 61.5%
78
Percent Composition
What is the percent carbon in C5H8NO4 (the
glutamic acid used to make MSG
monosodium glutamate), a compound used
to flavor foods and tenderize meats?
a) 8.22 %C
b) 24.3 %C
c) 41.1 %C
• Classwork p 344 #54-7
Empirical and molecular formulas
 If the identities of the elements of a
compound are known and you also know its
percentage composition, you can find the
formula of the compound.
Types of Formulas
 Empirical Formula
The formula of a compound that expresses the
smallest whole number ratio of the atoms present.
Ionic compounds formula are always empirical
formula
 Molecular Formula
The formula that states the actual number of
each kind of atom found in one molecule of the
compound.
Empirical Formula from % Composition
A compound analyzed and found to contain 25.9%
nitrogen and 74.1% O. What is the empirical formula
of the compound?
•Since percent means parts per 100, you can
assume 100.g of the compound contains 25.9g N
and 74.1 g O.
Empirical Formula from % Composition
A compound analyzed and found to contain 25.9%
nitrogen and 74.1% O. What is the empirical formula
of the compound?
•Convert those values to moles:
25.9 g N
1 mol N
= 1.85 mol N
14.0 g N
74.1 g O
1 mol O = 4.63 mol O
16.0 g O
Empirical Formula from % Composition
A compound analyzed and found to contain 25.9%
nitrogen and 74.1% O. What is the empirical formula
of the compound?
•Divide each molar quantity by the smaller number of
moles.
1.85 mol N
= 1
1.85
4.63 mol O
1.85
= 2.50 mol O
Empirical Formula from % Composition
A compound analyzed and found to contain 25.9%
nitrogen and 74.1% O. What is the empirical formula
of the compound?
•The subscript for O is still NOT a whole number.
Multiply both numbers by the smallest whole number
that will convert both subscripts to whole numbers
1 mol N x 2 = 2 mol N
2.50 mol O x 2 = 5 mol O
Empirical Formula: N2O5
A sample of a brown gas, a major air pollutant, is found
to contain 2.34 g N and 5.34g O. Determine the
empirical formula for the compound.
•convert grams to moles
moles of N = 2.34g of N 1 mol
= 0.167 moles of N
14.0 g
moles of O = 5.34 g
1 mol = 0.334 moles of O
16.0 g
• divide by smaller mole value:
0.167 mol N = 1 mol N
0.334 mol O = 2 mol O
0.167
0.167
Empirical formula : NO2
• CW p346 #58-61
Molecular Formula
• Some elements combine in more than one
way. The molecular formula of a compound is
either its empirical formula or a wholenumber multiple of its empirical formula.
Calculate the molecular formula of a compound whose
molar mass is 60.0g/mol and empirical formula is CH4N.
• Calculate molar mass of empirical formula:
Molar mass CH4N: 30.0 g/mol
• Divide molar mass molecular formula by
molar mass empirical formula:
60.0 g/mol = 2
30.0 g/mol
• Multiply the subscripts by this value.
Molecular formula: C2H8N2
The compound methyl butanoate smells like apples. Its
percent composition is 58.8% C, 9.8%H, and 31.4% O. Its
molar mass is 102 g/mol. What is its empirical formula? Its
molecular formula?
• Calculate empirical formula:
•
•
•
•
•
•
•
C5H10O2
Calculate molar mass empirical formula:
102 g/mol
Divide
102/102= 1
Multiply subscripts
C5H10O2
• Cw p 350 #62-66
Hydrates
• Hydrates are compounds that have a specific
number of water molecules bound to them.
• Naming hydrates:
– Calcium chloride dihydrate: CaCl2 2H2O
– Prefixes table 10.1 p351
Water
molecules
1
2
3
prefix
Mono- Di- Tri-
4
5
Tetra- Penta-
6
7
8
Hexa-
Hepta- Octa-
9
10
Nona- Deca-
• When a hydrate is heated, water evaporates leaving
the anhydrous (without water) compound
• Uses:
– Dessicators: absorb moisture from air
– Store solar energy
Ex.1 A mass of 2.50g of blue, hydrated copper (II)
sulfate (CuSO4  x H2O) is heated. After heating,
1.59g of anhydrous copper(II) sulfate (CuSO4)
remains. What is the formula and the name for the
hydrate?
Ex. 2 Cerium (III) iodide (CeI3) occurs as a hydrate with
the composition 76.3% CeI3 and 23.7% H2O.
Calculate the formula for the hydrate.
Cw. P361 #184-187

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