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CHEMICAL EQUATIONS
DOE-HDBK-1015/1-93
Fundamentals of Chemistry
CH-01
Rev. 0
Page 44
The equation weight in grams of a compound or element is defined as the gram molecular weight
times the number of molecules of the compound, as shown by the coefficients of the chemical
equation for the reaction. The sums of the equation weights on each side of a chemical equation
must be equal. Chemical calculations are based on the fact that every fraction or multiple of the
equation weights of substances that react gives a corresponding fraction or multiple of the
equation weights of the products of the reaction. In other words, if 30 grams of a substance that
has an equation weight of 15 grams reacts with some amount of another substance to form a
product with an equation weight of 20 grams, then 40 grams of that product will be formed.
Example:
How many grams of ferric oxide will be formed if 27.9 grams of iron reacts with water
according to the following equation.
Solution:
The equation weight of iron equals the gram atomic weight of iron times the number of
atoms shown reacting in the equation, which is two. Using Table 2:
Because 27.9 g of iron react, the fraction of the equation weight that reacts is:
.
Thus, 1/4 of the equation weight of ferric oxide will be formed.
The equation weight of ferric oxide equals the gram molecular weight of ferric oxide
times the number of molecules shown formed in the equation, which is one.
Using Table 2:
Thus, the amount of ferric oxide formed is:
.
