SCH4C – Chemical Calculations
533583498
Date: ________________________
Percent Composition
It is often useful to know the composition of a compound in terms of _____________
by mass of the _____________ making up the compound
Example: If a farmer wishes to put fertilizer on a field, the farmer may be interested
in knowing the amount of nitrogen present in a given amount of the fertilizer,
ammonia.
Example: What is the percentage composition of strontium nitrate?
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SCH4C – Chemical Calculations
533583498
Example: Indigo C16H10N2O2 is a blue dye. Determine the percentage composition
of indigo.
Example: How much lithium is in a 157 g sample of lithium chlorate?
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SCH4C – Chemical Calculations
533583498
Practice Problems
1. A sample of a liquid with a mass of 8.657 grams was decomposed into its elements
and gave 5.217 grams of carbon, 0.9620 grams of hydrogen, and 2.478 grams of
oxygen. What is the percentage composition of this compound?
2. A 27.0 g sample of a compound contains 7.20 g of C, 2.20 g of hydrogen and 17.6 g
of oxygen. Calculate the percentage composition of the compound.
3. In one sample of a compound (A) of copper and oxygen, 3.12 g of the compound
contains 2.50 g of copper and the remainder is oxygen. In another sample of a
compound (B) of copper and oxygen, 1.62 g of the compound contains 1.44 grams
of copper and the remainder is oxygen. Are these two samples the same
compound? Justify your answer.
4. Calculate the percentage by mass of the following compounds.
a) MgCl2
b) Na2SO4
c) Fe2O3
d) C7H5N3O6
e) AlBr3 • 6 H2O
5. Calculate the percentage of nitrogen in the following fertilizers.
a) ammonia, NH3
b) urea, CO(NH2)2
6. What is the percent composition of calcium oxide?
7. Determine the percentage composition of calcium phosphate.
8. The drug known as LSD has the formula C20H25N3O. One suspected sample
contained 74.07%C, 7.95%H, and 9.99% N, with the rest being oxygen. Are the
percentages given in the question consistent for LSD within reasonable limits of
error and rounding?
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SCH4C – Chemical Calculations
533583498
Answers:
1.
C
H
O
60.26%
11.11%
28.62%
2.
C
H
O
26.7%
8.1%
65.2%
3.
A Cu
80.1%
O
19.9%
B Cu
88.9%
O
11.1%
Not the same compound
4.
a) Mg
Cl
b) Na
S
O
c) Fe
O
d) C
H
N
O
e) Al
Br
H
O
25.53%
74.47%
32.37%
22.58%
45.05%
69.94%
30.06%
36.99%
2.22%
18.51%
42.28%
7.199%
63.95%
3.234%
25.61%
5.
a) N
b) N
82.22%
46.65%
6. CaO
Ca
O
71.47%
28.53%
7. Ca3(PO4)2
Ca
38.765%
P
19.97%
O
41.266%
8. C
74.255%
H
7.806%
N
12.99%
O
4.946%
Percent composition does not match
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