Law of multiple proportions
Encyclopedia
In chemistry
, the law of multiple proportions is one of the basic laws
of stoichiometry
, alongside the law of definite proportions
. It is sometimes called Dalton's Law after its discoverer, the English
chemist John Dalton
.
The statement of the law is:
If two element
s form more than one compound
between them, then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers.
For example carbon oxide: CO
and CO2
, 100 grams of carbon
may react with 133 grams of oxygen
to produce carbon monoxide
, or with 266 grams of oxygen to produce carbon dioxide
. The ratio of the masses of oxygen that can react with 100 grams of carbon is 266:133 ≈ 2:1, a ratio of small whole numbers. John Dalton specified that the law did not have to be followed in regards to just two of the same elements forming, following the example given above, this would mean that NO2 and NO would follow the law, but these two compounds cannot because of their multiple valence states. The Law of Multiple Proportions, is just what the name suggests, the law of multiple proportions of one constant element within differing compounds sharing the same type of chemical bonding.
John Dalton first expressed this observation in 1803. A few years previously, the French
chemist Joseph Proust
had proposed the law of definite proportions, which expressed that the elements combined to form compounds in certain well-defined proportions, rather than mixing in just any proportion. Careful study of the actual numerical values of these proportions led Dalton to propose his law of multiple proportions. This was an important step toward the atomic theory
that he would propose later that year, and it laid the basis for chemical formula
s for compounds.
The law of multiple proportions is best demonstrated using simple compounds. For example, if one tried to demonstrate it using the hydrocarbons decane
(chemical formula C10H22) and undecane
(C11H24), one would find that 100 grams of carbon could react with 18.46 grams of hydrogen to produce decane or with 18.31 grams of hydrogen to produce undecane, for a ratio of hydrogen masses of 121:120, which is hardly a ratio of "small" whole numbers.
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
, the law of multiple proportions is one of the basic laws
Chemical law
Chemical laws are those laws of nature relevant to chemistry. The most fundamental concept in chemistry is the law of conservation of mass, which states that there is no detectable change in the quantity of matter during an ordinary chemical reaction...
of stoichiometry
Stoichiometry
Stoichiometry is a branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions. In a balanced chemical reaction, the relations among quantities of reactants and products typically form a ratio of whole numbers...
, alongside the law of definite proportions
Law of definite proportions
In chemistry, the law of definite proportions, sometimes called Proust's Law, states that a chemical compound always contains exactly the same proportion of elements by mass. An equivalent statement is the law of constant composition, which states that all samples of a given chemical compound have...
. It is sometimes called Dalton's Law after its discoverer, the English
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
chemist John Dalton
John Dalton
John Dalton FRS was an English chemist, meteorologist and physicist. He is best known for his pioneering work in the development of modern atomic theory, and his research into colour blindness .-Early life:John Dalton was born into a Quaker family at Eaglesfield, near Cockermouth, Cumberland,...
.
The statement of the law is:
If two element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
s form more than one compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
between them, then the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers.
For example carbon oxide: CO
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
and CO2
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, 100 grams of carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
may react with 133 grams of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
to produce carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
, or with 266 grams of oxygen to produce carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
. The ratio of the masses of oxygen that can react with 100 grams of carbon is 266:133 ≈ 2:1, a ratio of small whole numbers. John Dalton specified that the law did not have to be followed in regards to just two of the same elements forming, following the example given above, this would mean that NO2 and NO would follow the law, but these two compounds cannot because of their multiple valence states. The Law of Multiple Proportions, is just what the name suggests, the law of multiple proportions of one constant element within differing compounds sharing the same type of chemical bonding.
John Dalton first expressed this observation in 1803. A few years previously, the French
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
chemist Joseph Proust
Joseph Proust
Joseph Louis Proust was a French chemist.-Life:Joseph L. Proust was born on September 26, 1754 in Angers, France. His father served as an apothecary in Angers. Joseph studied chemistry in his father’s shop and later came to Paris where he gained the appointment of apothecary in chief to the...
had proposed the law of definite proportions, which expressed that the elements combined to form compounds in certain well-defined proportions, rather than mixing in just any proportion. Careful study of the actual numerical values of these proportions led Dalton to propose his law of multiple proportions. This was an important step toward the atomic theory
Atomic theory
In chemistry and physics, atomic theory is a theory of the nature of matter, which states that matter is composed of discrete units called atoms, as opposed to the obsolete notion that matter could be divided into any arbitrarily small quantity...
that he would propose later that year, and it laid the basis for chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
s for compounds.
The law of multiple proportions is best demonstrated using simple compounds. For example, if one tried to demonstrate it using the hydrocarbons decane
Decane
Decane is an alkane hydrocarbon with the chemical formula CH38CH3.75 structural isomers of decane exist, all of which are flammable liquids. Decane is one of the components of gasoline . Like other alkanes, it is nonpolar and therefore will not dissolve in polar liquids such as water...
(chemical formula C10H22) and undecane
Undecane
Undecane is a liquid alkane hydrocarbon with the chemical formula CH39CH3. It is used as a mild sex attractant for various types of moths and cockroaches, and an alert signal for a variety of ants. It has 159 isomers....
(C11H24), one would find that 100 grams of carbon could react with 18.46 grams of hydrogen to produce decane or with 18.31 grams of hydrogen to produce undecane, for a ratio of hydrogen masses of 121:120, which is hardly a ratio of "small" whole numbers.