Stoichiometry Worksheet Page 4
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6A favorite of mine and one that sometimes comes up on the AP exam is combustion analysis. To solve a
combustion analysis problem one must know that all of the carbon in a compound is converted to carbon dioxide
and all of the hydrogen in a compound is converted to water. Usually the percent analysis is given for any
additional element, except oxygen. To find the mass of oxygen you must subtract the masses of the other elements
from the total. Once you know the mass of each element you can find the empirical and molecular formulas. Try
the one below; the answers are given but the work is not.
Answer the following questions that relate to the analysis of chemical compounds.
A compound containing the elements C, H, N, and O is analyzed. When a 1.2359 g sample is burned in excess
oxygen, 2.241 g of CO
(g) is formed. The combustion analysis also showed that the sample contained 0.0648 g of H.
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(i) Determine the mass, in grams, of C in the 1.2359 g sample of the compound. 0.6116 g C
(ii) When the compound is analyzed for N content only, the mass percent of N is found to be 28.84 percent.
Determine the mass, in grams, of N in the original 1.2359 g sample of the compound. 0.3564 g N
0.2031 g O
(iii) Determine the mass, in grams, of O in the original 1.2359 g sample of the compound.
(iv) Determine the empirical formula of the compound. C
H
N
O
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(v) The molecular mass of the compound is 194.2 g/mol. Determine the molecular formula of the
compound. C
H
N
O
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Chemical Equations
Chemical changes are represented by chemical reactions.
In a chemical reaction, the matter is not created or destroyed (Law of Conservation of Mass). Instead
the atoms in the chemical reaction are rearranged. A balanced equation has equal numbers of each type
of atom on each side of the equation.
coefficient
coefficient
Coefficients - the numbers written in front of the chemical formulas
used in balancing chemical equations.
2 H
2 H
+ O
O
Reactants - the elements or compounds on the left hand side of the
2
2
2
equation before the are known as the reactants.
Products - the elements or compounds on the right side of the
equation after the are known as the products.
Reactants
Products
Yield() - this arrow is called a yield sign. It separates the reactants
yield
from the products.
The word diatomic is used above when we read the equation. A diatomic molecule is
Diatomic Elements
made up of two atoms of the same element. These elements never exist as one
H
N
O
naturally. When we are writing chemical equations you must always remember to write
2
2
2
F
Cl
Br
I
a subscript of two when these elements are by themselves. (When they are bonded to
2
2
2
2
other elements they do not have to have a subscript of 2.) There are seven diatomic
elements; they are listed in the chart to the right. You must MEMORIZE these seven diatomic elements. You can
see that they kind of form a pattern on the periodic table. Many pneumonic exist. Ask me if you need one.
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