Importance of the Law of Definite Composition

The law of definite composition explained

The law of definite composition says that a given chemical compound always contains the same elements in the same fixed proportions by weight. This is important if you want to make a compound.

Atomic Weight

On the periodic table, each element has an atomic weight. These numbers describe the relative weights of the elements. Hydrogen is approximately 1, carbon is approximately 12 and oxygen is approximately 16. These numbers show the weight ratios of the elements that make up a compound.

Decomposing a Compound

The chemical formula for water is H2O, or two hydrogen atoms for every oxygen atom. If 18 grams of water is disassembled, it would produce 2 grams of hydrogen (the atomic weight of hydrogen is 1, and there are 2 of them) and 16 grams of oxygen (there is only one oxygen atom for each water molecule, but it has weight of 16).

Assembling a Compound

To find the amounts of each element that goes into making up a compound, the law of definite composition is applied backward. Two pounds of hydrogen and 16 pounds of oxygen are required to make 18 pounds of water. The law of definite composition is important to assemble or dismantle chemical compounds.

References

• PTable: Periodic Table of Elements
• JCCC: Basic Chemistry Concepts
• ConvertUnits: Molecular Weight

Resources

• QA02: The Law of Definite Composition