Percentage Composition and Empirical Formulas percent composition can be determined either by...
Transcript of Percentage Composition and Empirical Formulas percent composition can be determined either by...
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Percentage
Composition and
Empirical Formulas
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The formula for a compound indicates the number and kind of each atom in a representative particle of the compound.
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How many atoms of each element are found in the following formula?
Al2(CrO4)3
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Percentage Composition
The percentage composition of a compound gives the percent of the total mass made up by each element in the compound.
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The percent composition can be determined either by calculating percentage composition from a givenchemical formula or by experimental decomposition and analysis of a compound.
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Example
Ammonia has the formula NH3. Use the formula to determine the percentage composition of ammonia.
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Step 1
Find the mass of one mole of chemical formula.
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Step 1
Mass of nitrogen
1 mole N 14 g = 14 g
mole
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Step 1
Mass of hydrogen
1 mole H 1 g = 1 g x 3
mole
3 g
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Step 1
Total Mass of NH3
14 g + 3 g = 17 g
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Step 2
Find the percent of each element
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Step 2
% element =
Mass of element in compound X 100%
Total Mass of compound
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Step 2
% nitrogen =
14 g X 100% = 82 %
17 g
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Step 2
% hydrogen =
3 g X 100% = 18 %
17 g
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Determining an Empirical
Formula given Percentage
Composition
A formula which gives the simplest whole number ratio of atoms (elements) of a compound.
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Example
Determine the formula for the compound that when analyzed showed 70.9% potassium and 29.1% sulfur by weight.
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Step 1
Percentage by mass may be assumed to be grams of element per 100 grams of compound
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Step 1
Thus, in every 100 grams of the compound there is
70.9 g of potassium
29.1 g of sulfur
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Step 2
Because potassium and sulfur do not the same atomic mass the ratio of the masses does not indicate their mole relationships.
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Step 2
Divide the percent (now considered grams) of each element by its molar mass.
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Step 2
K = 70.9 g mole =
39 g
1.82 mole
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Step 2
S = 29.1 g mole =
32 g
.91 mole
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Step 3
Atoms combine in small whole number ratios, however, so these fractional values must be converted to whole numbers.
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Step 3
Divide each number by the smallest one.
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Step 3
K = 1.82 mole = 2
.91 mole
S = .91 mole = 1
.91 mole
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Step 3
The simplest formula must then be
K2S
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Note:
A general rule to follow in these problems is to place the elements in the formula in the same sequence given in the problems.
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Determining the Molecular
Formula from the Empirical
Formula
The empirical formula of a compound indicates the simplest ratio of the atoms in the compound.
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It does not indicate to actual numbers of atoms in each molecule of the compound.
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The molecular formula gives the actual number of atoms of each element in a molecule. It is always a whole number multipleof the empirical formula.
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Example:
Analysis of a compound showed it to consist of 80% carbon and 20% hydrogen by mass. The molecular mass was found to be 30g. What is the molecular formula?
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Step 1
Find the empirical formula first.
Using the procedures above.
The empirical formula is CH3.
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Step 2
Determine the molecular formula from the molecular mass and the formula mass.
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Step 2
Empirical formula mass
12 + (1 X 3) = 15 g
(calculated from CH3)
Molecular mass = 30 g
(given in the problem)
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Step 2
Number of multiples =
molecular mass
empirical formula mass
30 g = 2
15 g
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Step 2
molecular formula is
(CH3)2 or C2H6