1 chapter 6 chemical quantities 6.5 percent composition and empirical formulas copyright © 2008 by...
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1
Chapter 6 Chemical Quantities
6.5 Percent
Composition and
Empirical Formulas
Copyright © 2008 by Pearson Education, Inc.Publishing as Benjamin Cummings
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Percent Composition
Percent composition
• Is the percent by mass of each element in a formula.
Example: Calculate the percent composition of CO2.
CO2 = 1 C(12.01g) + 2 O(16.00 g) = 44.01 g/mol)
12.01 g C x 100 = 27.29 % C 44.01 g CO2
32.00 g O x 100 = 72.71 % O 44.01 g CO2 100.00 %
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What is the percent composition of lactic acid, C3H6O3, a compound that appears in the blood after vigorous activity?
Learning Check
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STEP 1
3C(12.01) + 6H(1.008) + 3O(16.00) = 90.08 g/mol
36.03 g C + 6.048 g H + 48.00 g O
STEP 2
%C = 36.03 g C x 100 = 40.00% C 90.08 g
%H = 6.048 g H x 100 = 6.714% H 90.08 g
%O = 48.00 g O x 100 = 53.29% O 90.08 g
Solution
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Learning Check
The chemical isoamyl acetate C7H14O2 gives the odor of pears. What is the percent carbon in isoamyl acetate?
1) 7.102 %C
2) 35.51 %C
3) 64.58 %C Copyright © 2008 by Pearson Education, Inc.Publishing as Benjamin Cummings
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3) 64.58 %C
Molar mass C7H14O2 = 7C(12.01) + 14H(1.008)
+ 2O(16.00) = 130.18 g/mol
Total C = 7C(12.01) = g
% C = total g C x 100 total g
% C = 84.07 g C x 100 = 64.58 % C 130.18 g
Solution
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The empirical formula
• Is the simplest whole number ratio of the atoms.
• Is calculated by dividing the subscripts in the actual (molecular) formula by a whole number to give the lowest ratio.
C5H10O5 5 = C1H2O1 = CH2Oactual (molecular) empirical formula
formula
Empirical Formulas
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Some Molecular and Empirical Formulas
• The molecular formula is the same or a multiple of the empirical.
Table 6.3
Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings
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A. What is the empirical formula for C4H8?
1) C2H4 2) CH2 3) CH
B. What is the empirical formula for C8H14?
1) C4H7 2) C6H12 3) C8H14
C. Which is a possible molecular formula for CH2O?
1) C4H4O4 2) C2H4O2 3) C3H6O3
Learning Check
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A. What is the empirical formula for C4H8?
2) CH2 C4H8 4
B. What is the empirical formula for C8H14?
1) C4H7 C8H14 2
C. Which is a possible molecular formula for CH2O?
2) C2H4O2 3) C3H6O3
Solution
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A compound has an empirical formula SN. If there are 4 atoms of N in one molecule, what is the molecular formula? Explain.
1) SN
2) SN4
3) S4N4
Learning Check
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A compound has an empirical formula SN. If there are 4 atoms of N in one molecule, what is the molecular formula? Explain.
3) S4N4
In this molecular formula 4 atoms of N and 4 atoms of S and N are related 1:1. Thus, it has an empirical formula of SN.
Solution
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A compound contains 7.31 g Ni and 20.0 g Br. Calculate its empirical (simplest) formula.
Learning Check
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Convert 7.31 g Ni and 20.0 g Br to moles.
7.31 g Ni x 1 mol Ni = 0.125 mol Ni
58.69 g Ni
20.0 g Br x 1 mol Br = 0.250 mol Br
79.90 g Br
Divide by smallest:
0.125 mol Ni = 1 Ni 0.250 mol Br = 2 Br
0.125 0.125
Write ratio as subscripts: NiBr2
Solution
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Converting Decimals to Whole Numbers
When the number of moles for an element is adecimal, all the moles are multiplied by a small
integer to obtain whole number.
Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings
Table 6.4
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Aspirin is 60.0% C, 4.5 % H and 35.5 % O. Calculate its empirical (simplest) formula.
Learning Check
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STEP 1. Calculate the moles of each element in 100 g.
100 g aspirin contains 60.0% C or 60.0 g C, 4.5% H or 4.5 g H, and 35.5% O or 35.5 g O.
60.0 g C x 1 mol C = 5.00 mol C 12.01 g C
4.5 g H x 1 mol H = 4.5 mol H 1.008 g H
35.5 g O x 1mol O = 2.22 mol O
16.00 g O
Solution
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Solution (continued)
STEP 2. Divide by the smallest number of mol.
5.00 mol C = 2.25 mol C (decimal)
2.22 4.5 mol H = 2.0 mol H2.222.22 mol O = 1.00 mole O2.22
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Solution (continued)
3. Use the lowest whole number ratio as subscripts
When the moles are not whole numbers, multiply by a factor to give whole numbers, in this case x 4.
C: 2.25 mol C x 4 = 9 mol CH: 2.0 mol H x 4 = 8 mol HO: 1.00 mol O x 4 = 4 mol O
Using these whole numbers as subscripts the simplest formula is
C9H8O4
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