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Law of Multiple Proportions

Elements combine in the ratio of small whole numbers.

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Law of Multiple Proportions

Unicycles and bicycles have different numbers of wheels

Credit: Steve
Source: http://commons.wikimedia.org/wiki/File:Five_Boro_Bike_Tour_unicycle.jpg
License: CC BY-NC 3.0

What are the similarities and differences between a unicycle and a bicycle? 

Just from the words themselves, the astute Latin-speaking scholar can tell that, whatever it is made of, the unicycle has one of them (uni = “one”) and the bicycle has two (bi = “two”). From the picture above, we get additional information that helps us tell the two apart.  The unicycle has one wheel and the bicycle has two. In particular, they are made up of the same materials and the only significant difference is the number of wheels on the two vehicles.  So now, how many wheels on a tricycle?

Law of Multiple Proportions

Once the idea that elements combined in definite proportions to form compounds was established, experiments also began to demonstrate that the same pairs of certain elements could combine to form more than one compound.  Consider the elements carbon and oxygen.  Combined in one way, they form the familiar compound called carbon dioxide.  In every sample of carbon dioxide, there is 32.0 g of oxygen present for every 12.0 g of carbon.  By dividing 32.0 by 12.0, this simplifies to a mass ratio of oxygen to carbon of 2.66 to 1.  There is another compound that forms from the combination of carbon and oxygen called carbon monoxide.  Every sample of carbon monoxide contains 16.0 g of oxygen for every 12.0 g of carbon.  This is a mass ratio of oxygen to carbon of 1.33 to 1.  In the carbon dioxide, there is exactly twice as much oxygen present as there is in the carbon monoxide.  This example illustrates the law of multiple proportions:  Whenever the same two elements form more than one compound, the different masses of one element that combine with the same mass of the other element are in the ratio of small whole numbers.

Carbon can react with oxygen to form carbon monoxide or carbon dioxide

Credit: CK-12 Foundation - Christopher Auyeung
License: CC BY-NC 3.0

Carbon can form two different compounds with oxygen.[Figure2]

In carbon monoxide, on the left, there is 1.333 g of oxygen for every 1 g of carbon.  In carbon dioxide, on the right, there is 2.666 g of oxygen for every gram of carbon.  So the ratio of oxygen in the two compounds is 1:2, a small whole number ratio.

The difference between carbon monoxide and carbon dioxide is significant.  Carbon monoxide is a deadly gas, formed from the incomplete combustion of some carbon-containing materials (such as wood and gasoline).  This compound will attach to hemoglobin in the red blood cell and block the binding of oxygen to those cells.  If oxygen does not bind, it cannot be carried to the cells of the body where it is needed, and death can occur.  Carbon dioxide, on the other hand, is not toxic like carbon monoxide is.  However, it can displace oxygen in systems since it is heavier.  Carbon dioxide fire extinguishers cut off the flow of oxygen in a fire, putting out the fire.


  • The law of multiple proportions states that whenever the same two elements form more than one compound, the different masses of one element that combine with the same mass of the other element are in the ratio of small whole numbers.


  1. State the law of multiple proportions.
  2. In carbon dioxide, how many grams of oxygen would there be if there are 24 grams of carbon?
  3. How many grams of carbon would be present in carbon monoxide that contains 2.66 grams of oxygen?

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Image Attributions

  1. [1]^ Credit: Steve; Source: http://commons.wikimedia.org/wiki/File:Five_Boro_Bike_Tour_unicycle.jpg; License: CC BY-NC 3.0
  2. [2]^ Credit: CK-12 Foundation - Christopher Auyeung; License: CC BY-NC 3.0

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