Percent Composition
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Transcript Percent Composition
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