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Lecture 2
Empirical and Molecular Formulae
IB Chemistry Power Points
Topic 1
Quantitative Chemistrywww.pedagogics.ca
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Percent Composition by Mass
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Recall: the relative molecular mass of H2SO4 can be determined as follows:
Hydrogen 2 atoms × 1.008 = 2.01Sulfur 1 atom × 32.07 = 32.07Oxygen 4 atoms × 16.000 =64.00
MR 98.09
The percent composition by mass states the fraction of the total mass of each element in the compound.
For example % sulfur
32.07% 100 32.7%
98.09S
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Similarly:
2.016% 100 2.1%
98.09H
64.00% 100 65.2%
98.09O
Note the sum of these values should add to 100%
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Empirical Formula Determination
That is “empirical” not Empire you fool!
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Definition
The empirical formula is the formula that gives
the simplest whole number ratio of atoms in
a compound
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An example
Adipic acid contains 49.32% C, 43.84% O, and 6.85% H by mass. What is the empirical formula of adipic acid?
49.32 14.107
12.01
g C mol Cmol C
g C
Come to the dark
side
1. Assume 100g of the compound
2. So, in 100 g of adipic acid, there would be 49.32 g of C, 43.84 g of O and 6.85 g of H.
3. Determine moles of each element. For example: carbon
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An example continued
H mol 78.6mol g 1.01
H g 85.61-
Similarly:
I’m your father!
O mol 74.2mol g 16.00
O g 84.431-
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Step 4. Divide each value of moles by the smallest of the values.
1 2.74 mol 74.2 :
2.5 2.74 mol 78.6 :
1.5 2.74 mol .124 :
Oxygen
Hydrogen
Carbon
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Step 5. If necessary, multiply each number by an integer to obtain all whole numbers.
Carbon: 1.5 Hydrogen: 2.5 Oxygen: 1x 2 x 2 x 2
3 5 2
Empirical formula:C3H5O
2
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Alternatively, the empirical formula can be determined from mass composition data.
Using the same example: a 21.6 g sample of adipic acid is decomposed yielding some carbon, 9.5 g of oxygen gas and 1.5 g of hydrogen gas. What is the empirical formula of adipic acid?
First find the number of MOLES of each elementThen determine the lowest whole number ratio for the mole values
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A MOLECULAR FORMULA is the actual chemical formula of a compound. The molecular formula will contain elements in the same ratio as the empirical formula.
Molecular formulae are whole number multiples of empirical formulae.
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We have determined that the empirical formula for adipic acid is C3H5O2. If the molar mass of adipic acid is 146 g/mol, what is the molecular formula of adipic acid?
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1. Find the molar mass of C3H5O2
3(12.01 g) + 5(1.01) + 2(16.00) = 73.08 g
2. Divide the molecular mass by the mass given by the empirical formula.
146 ÷ 73.08 = 2
3. Multiply the empirical formula by this number to get the molecular formula.
(C3H5O2) x 2 = C6H10O4