Stoichiometry Chemistry Worksheets With Answers Page 2
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3
4
5
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10
11Oxygen, O
32.00
2.00
64.0
2 4
2
1.20
10
Methane, CH
16.05
0.279
4.48
2 3
4
1.68
10
Nitrogen, N
28.02
158.00
4430
2 5
2
9.50
10
Now that you’ve had some practice figuring out molecular weights, let’s talk about how you’ll be
expected to use them, and other stoichiometric tools, on the exam. For example, you will almost
certainly be asked to find the percent composition of a compound, so let’s talk about that first.
Percent Composition of Compounds
There are two different ways to describe the composition of a compound: in terms of the number of
its constituent atoms (like C
H
) and in terms of the percentages (by mass) of its elements. When
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6
showing the constituent atoms of a molecule, you can either show the chemical formula, which
shows the real number of atoms in the molecule, like C
H
, or show the empirical formula, which
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6
merely shows their relative amounts in a substance, so the above molecular formula would be
expressed as CH
.
3
You can describe the composition of a compound in terms of the weights of its constituent elements
by determining the percent composition of particular elements in the molecule. To calculate percent
compositions, you would find the weight of each constituent atom, then figure out what percent of
the total molecular weight it makes up. Consider ethanol, C
H
OH. Taking subscripts into
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5
consideration, you have 2 mols of carbon, 6 mols of hydrogen (5 + 1), and 1 mol of O. Now convert
moles into grams for each constituent element as well as for the entire molecule:
Mass of C = 2
12.01
= 24.02 g
Mass of H = 6
1.01
= 6.06 g
Mass of O = 1
16.00
= 16.00 g
Mass of 1 mol of C
H
OH = 46.08 g
2
5
Now use the formula you learned above to find the percent compositions of the constituent elements:
Mass percent of C:
100% = 52.14%
Mass percent of H:
100% = 13.15%
Mass percent of O:
100% = 34.77%
Not so bad, right?
More Complex Stoichiometric Calculations
When you’re asked to do stoichiometric calculations on the SAT II Chemistry exam, make sure that if
you need to write out the chemical formulas, you do this correctly. No matter how good you are at
math and how well you understand the stoichiometric rules that follow, you won’t get the right
answer if your chemical formulas are wrong! If you feel that you’re weak in this area, see the review
(in Appendix II) of chemical formula naming and writing.
Perhaps the easiest way to approach problems that ask you to calculate the amounts of reactants
consumed or products produced during the course of a reaction is to start by creating a table or
chart. Let’s work through a typical example. Say the SAT II Chemistry test asks you what mass of
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