2/9/2015 1 Mass of Part / Mass of Whole Unit 9: Chemical Formulas Lesson 9.2: Percent Composition, Empirical & Molecular Formulas 40 Percent Composition • Sometimes it’s useful to know the composition of a compound in terms of what percentage of the total is each element • Percent – “Parts per 100” – The number of specific items per a group of 100 items – 50% of $100 is $50 (50 items/100 total items) Percent Example • You have 4 oranges and 5 apples. What percent of the total is oranges? • In “parts per 100 (cent)” oranges 44% 100% total 9 oranges 4 oranges 44% 100% total 100 oranges 44
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• Sometimes it’s useful to know the composition of a compound in terms of what percentage of the total is each element
• Percent – “Parts per 100”
– The number of specific items per a group of 100 items
– 50% of $100 is $50 (50 items/100 total items)
Percent Example
• You have 4 oranges and 5 apples. What percent of the total is oranges?
• In “parts per 100 (cent)”
oranges 44%100% total9
oranges 4
oranges 44%100% total100
oranges 44
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Percent Composition – In Chemistry
• It is the percent by mass of each element in a compound
• Can be determined
– By its chemical formula
– Molar masses of the elements that compose the compound
• The percent of each element contributes to the mass of the compound
100%
compoundtheofmassmolar
elementeachofmasselementeach of mass %
compound in the
Calculating Percent Composition from a Formula
• What is the percent composition of each element in NH4OH?
g14.01 = g14.011:N
g5.04 = g0078.15:H
g16.00 = g00.611:O
g35.05 =
%100g35.05
g14.01 :N
%100g35.05
g5.04 :H
%100g35.05
g16.00 :O
N 39.97%
H 14.38%
O 45.65%
Determine the contribution of each
element
Molar mass
Calculating Percent Composition from a Sample
• Experiments show that the 80.0 grams of a compound is made up of 59.53 grams of oxygen and 20. 47 grams of nitrogen. What is the % composition, by mass, of each element in the compound??
%.
100000 g8
g20.47 :N
%100g 80.00
g59.53 :O
N 25.59%
O 74.41%
Determine the contribution of each
element
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Percent Composition What is the mass of nitrogen in 85.0 mg of the amino acid lysine, C6H14N2O2.
1. Molar Mass of C6H14N2O2 C = 6(12.01) = 72.06 H =14(1.01) = 14.14
MM = 146.22
28.02 g ___________ x 100% 146.22 g
= 19.16%
3. 0.1916 x 85.0 mg = 16.3 mg N
2.
N = 2(14.01) = 28.02 O = 2(16.00) = 32.00
1. What is the % composition, by mass, of each element in SiO2?
2. Analysis of 35.00 g sample of a substance determined that it contains 18.25 g carbon, 4.60 g hydrogen, and 12.15 g oxygen. What is the % composition, by mass, of each element in the compound?
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Learning Check!
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
Learning Check!
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Chemical Formulas of Compounds
• Formulas give the relative numbers of atoms or
moles of each element in a formula unit - always a
whole number ratio (the law of definite
proportions).
NO2 2 atoms of O for every 1 atom of N
1 mole of NO2 : 2 moles of O atoms to every 1
mole of N atoms
• If we know or can determine the relative number
of moles of each element in a compound, we can determine a formula for the compound.
Do These Make Sense?
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Coefficients Coefficients tell you how many moles of the compound you have.
So, you have to multiply all atoms in the compound by the coefficient to find the total amount of atoms.
1 is not written as a coefficient.
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How Do We Determine Formulas?
Qualitative Analysis:
Use a spectrometer to find the atoms in the compound.
Quantitative Analysis:
See how the compound behaves in chemical reactions and use our measurements to figure out the formula
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Types of Formulas 1. Molecular Formula
The formula that states the actual number of each kind of atom found in one molecule of the compound.
Covalent compounds only!
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Methane (CH4) Benzene (C6H6)
2. Empirical Formula
The formula of a compound that expresses the smallest whole number ratio of the atoms present.
All ionic compounds & network solids (the “formula unit”)
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Sodium Chloride (NaCl) Silicon Dioxide (SiO2)
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Empirical Formulas of Molecular Compounds
Molecular Compounds can also be stated as empirical formulas.