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Law of Definite Proportions

Compounds contain the same elements in the same proportion by mass.

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Law of Definite Proportions

Electricity must be a certain voltage

Credit: Courtesy of Staff Sgt. Christopher Flurry, U.S. Marine Corps
Source: http://commons.wikimedia.org/wiki/File:USMC-110609-M-FK922-004.jpg
License: CC BY-NC 3.0

We use electricity for many purposes, from cooking to powering our televisions to charging our cell phones.  Wherever we travel in the United States, we want electricity to be available.  What we also want (although we usually don’t think about it) is for the electricity supply to be the same wherever we go.  We want the same voltage (110 volts for the U.S.) to come from the outlet to whatever we plug in.  If the voltage is less, the system will not work.  If it is more, the equipment will be damaged.  We want a specific amount of voltage – no more and no less.

Law of Definite Proportions

The discovery that mass was always conserved in chemical reactions was soon followed by the law of definite proportions, which states that a given chemical compound always contains the same elements in the exact same proportions by mass.  As an example, any sample of pure water contains 11.19% hydrogen and 88.81% oxygen by mass.  It does not matter where the sample of water came from or how it was prepared.  Its composition, like that of every other compound, is fixed.

Water coming out of a faucet

Credit: Jenn Durfey
Source: http://www.flickr.com/photos/dottiemae/5202454566/
License: CC BY-NC 3.0


Another example is carbon dioxide.  This gas is produced from a variety of reactions, often by the burning of materials.  The structure of the gas consists of one atom of carbon and two atoms of oxygen. Carbon dioxide production is of interest in many areas, from the amount we breathe out to the amount of the gas produced by burning wood or fossil fuels.  By knowing the exact composition of carbon dioxide, we can make predictions as to the effects of different chemical processes.

Carbon dioxide is formed by burning wood

Credit: Courtesy of Patsy Lynch, FEMA
Source: http://commons.wikimedia.org/wiki/File:FEMA_-_23780_-_Photograph_by_Patsy_Lynch_taken_on_04-17-2006_in_Missouri.jpg
License: CC BY-NC 3.0

Carbon dioxide is produced during the burning process.[Figure3]


  • The law of definite proportions states that a given chemical compound always contains the same elements in the exact same proportions by mass.


  1. State the law of definite proportions.
  2. Will the composition of water vary depending on its source?
  3. Why is this law important?

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Use the resource below to answer the questions that follow.

  1. When was the law of definite proportions developed?
  2. Who proposed this law?
  3. How many hydrogen atoms are there in a molecule of water?
  4. How many oxygen atoms are there in a molecule of water?

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Image Attributions

  1. [1]^ Credit: Courtesy of Staff Sgt. Christopher Flurry, U.S. Marine Corps; Source: http://commons.wikimedia.org/wiki/File:USMC-110609-M-FK922-004.jpg; License: CC BY-NC 3.0
  2. [2]^ Credit: Jenn Durfey; Source: http://www.flickr.com/photos/dottiemae/5202454566/; License: CC BY-NC 3.0
  3. [3]^ Credit: Courtesy of Patsy Lynch, FEMA; Source: http://commons.wikimedia.org/wiki/File:FEMA_-_23780_-_Photograph_by_Patsy_Lynch_taken_on_04-17-2006_in_Missouri.jpg; License: CC BY-NC 3.0

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