The law of definite proportions states that any samples of a single compound contain the same proportion of elements by mass. Other names for the law are Proust’s law or the law of constant composition. Whichever name you use, this law together with the law of multiple proportions form the basis for stoichiometry in chemistry.
Examples of the Law of Definite Proportions
For example, the mass ratio between hydrogen and oxygen is the same in any sample of pure water (H2O). The molar mass of water is the mass of the hydrogen atoms added to the mass of the oxygen atoms. These numbers are on the periodic table.
mass of water = (2 x mass of hydrogen) + mass of oxygen = (2 x 1.01) + 16.00 = 18.02
Hydrogen accounts for 11.19% o the mass of the compound:
% hydrogen = 2.02/18.02 = 0.112 = 11.2%
% oxygen = 16.00/18.02 = 0.888 = 88.8%
Another way of looking at it is that water is always 1/9 hydrogen and 8/9 oxygen by mass.
The law of definition proportions applies to ionic compounds, too. For example, the formula for table salt or sodium chloride is NaCl. The atomic mass of sodium is ~23 and the mass of chlorine is ~35. Based on the mass ratio between the elements, you expect dissociating 58 grams of NaCl yields 23 grams of sodium and 35 grams of chlorine.
If two compounds consisting of the same elements have different mass ratios from one another, you know they are two different compounds. So, if you have samples with two formulas using the same elements, such as CO and CO2, you know they are completely different compounds.
Exceptions to the Law of Definite Proportions
The law of definite proportions has exceptions. Non-stoichiometric compounds vary in mass composition from one sample to the next. For example, the formula for the iron oxide mineral called wustite is Fe0.95O and not FeO because the crystal structure allows for slight deviations from the stoichiometric ratio.
Isotopes also influence the mass composition of a compound. The ratio of isotopes of a given element varies according to its source. This leads to samples from different locations having different element mass ratios. Usually, the mass difference is small, unless a compound contains hydrogen.
Polymers also vary in element composition by mass, depending on how many monomers they contain. However, their chemical formulas usually are stoichiometric ratios that adhere to the law of definite proportions.
The law of definition proportions is important in everyday chemistry calculations, but it also has historical significance. It was among the laws supporting John Dalton‘s 1803 atomic theory.
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