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4.4: Law of Definite Proportions

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     Electricity must be a certain voltage
    Figure \(\PageIndex{1}\) (Credit: Courtesy of Staff Sgt. Christopher Flurry, U.S. Marine Corps; Source: Commons Wikimedia, File:USMC-110609-M-FK922-004.jpg(opens in new window) []; License: Public Domain)

    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 definite 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 from the faucet.
    Figure \(\PageIndex{2}\): Water. (Credit: Jenn Durfey; Source: Flickr Photos, Water Faucet(opens in new window) []; License: CC by 2.0(opens in new window))

    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 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.

    Home burning down, with debris.
    Figure \(\PageIndex{3}\): Carbon dioxide is produced during the burning process. (Credit: Courtesy of Patsy Lynch, FEMA; Source: Commons Wikimedia, FEMA Photograph by Patsy Lynch taken on 04-17-2006 in Missouri(opens in new window) []; License: Public Domain)


    • 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 molecules vary depending on its source?
    3. Why is this law important?

    Explore More

    Use the resource below to answer the questions that follow.

    1. How many hydrogen atoms are there in a molecule of water?
    2. How many oxygen atoms are there in a molecule of water?
    3. What is the mass ratio of hydrogen to oxygen in a molecule of water?
    4. Will the mass ratio of hydrogen to oxygen change depending on the size of the sample?      For example if the sample size was 10 L or 10,000 L.

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