What is the law of multiple proportions?

Law of multiple proportions

The law of multiple proportions , which is also known as Dalton’s law, was proposed by the English chemist and meteorologist John Dalton in his work written in 1804 called A New System of Chemical Philosophy . The law is itself a rule of stoichiometry and was based on the observations made by Dalton with respect to the reactions of atmospheric gases , and which also establishes that when the elements form some type of compound , the proportionsof the elements that are present in these chemical compounds can be expressed in small proportions of whole numbers .

Law of multiple proportions

What is the law of multiple proportions?

The law of multiple proportions is a law enunciated by John Dalton , which tells us that when two elements can combine to form several compounds , the masses have a relationship of integers and simple numbers .

  • What is the law of multiple proportions
  • History
  • Who proposed it
  • Statement of the law of multiple proportions
  • Formula
  • Experiments
  • Applications of the law of multiple proportions
  • Importance
  • Examples

What is the law of multiple proportions

Just as different combinations are possible for example in food, it is also possible to make different combinations with the elements that make up the compounds . Remember that compounds are made up of atoms of different elements. There are some compounds made up of the same elements, such as carbon monoxide (CO) and carbon dioxide (CO2). Both compounds are made of carbon (C) atoms and oxygen (O) atoms; however, the proportions of carbon and oxygen in each compound are different . This illustrates the law of multiple proportions. The law of multiple proportions states that when two elementsThey are combined to form more than one compound , the mass having an element, which combines with a fixed mass of the other element, always be given in proportions of integers .


Although the concept of the atom was born with Democritus , the English meteorologist and chemist John Dalton enunciated the first modern description of the atom as the essential component of chemical structures. Dalton developed the law of multiple proportions by studying and expanding the works of Antoine Lavoisier and Joseph Proust .

Proust had studied tin oxides and found that their masses were 88.1% tin and 11.9% oxygen or 78.7% tin and 21.3%. Dalton observed from the percentages that 100 g of tin combined with 13.5 g or 27 g of oxygen; 13.5 and 27 form a 1: 2 ratio. He discovered that the atomic theory of matter could explain the common pattern in chemistry : In the case of Proust’s tin oxides, one tin atom will combine with one or two atoms. of oxygen.

Dalton also believed that atomic theory could explain why water absorbed different gases in different proportions, and he hypothesized that this was due to differences in mass and the complexity of the gas particles .

He proposed that each chemical element is composed of atoms of a unique and unique type, and although they cannot be altered or destroyed by chemical means, they can combine to form more complex structures forming chemical compounds . Since Dalton reached his conclusions by experimenting and examining the results in an empirical way, this marked the first truly scientific theory of the atom.

Who proposed it

The law of multiple proportions was enunciated by John Dalton , and he did so in the year 1803, and is considered an important stoichiometric law . It was demonstrated in practice by the French chemist Gay-Lussac.

Statement of the law of multiple proportions

The statement of the law of multiple proportions says the following:

“When two or more elements are combined to give more of a compound , a mass varying from one joins a mass fixed to the other, and the first has the relation numbers canonical and indistinct “.


The formula for the law of multiple proportions is as follows:

Pabs = #Ppi

Ppi = Pabs (% ix 100)


  • Pabs = absolute pressure of a gas
  • Ppi = partial pressure of a component of the mixture
  • #Ppi = Sum of the partial pressures of the gases that make up the mixture
  • % i = Percentage of gas in the mixture


An example of an experiment applying the law of multiple proportions would be:

S + O 2 -> SO 2      S + 3/2 O 2 -> SO 3

g of O = 16 * 2 g of O = 16 * 3

g of S = 32 g of S = 32

32 g of O react with 32 g of S to give SO 2              48 g of O react with 32 g of S to give SO 3

Applications of the law of multiple proportions

Some applications of the law are as follows:

  1. The partial pressure of oxygen and nitrogen at atmospheric pressure (1 ATA) will be:

PpO 2 = 21/100 x 1 = 0.21 atmospheres

PpN 2 = 79/100 x 1 = 0.79 atmospheres

The sum of the partial pressures is equal to the absolute pressure: 0.21 + 0.79 = 1 atmosphere .

  1. At a depth of 10 meters, where the absolute pressure is 2 ATA, the partial pressure of each air component will be:

PpO 2 = 21/100 x 2 = 0.42 atmospheres

PpN 2 = 79/100 x 2 = 1.58 atmospheres

PpO 2 + PpN 2 = 2 ATA


Dalton’s law of multiple proportions is part of the basis of the atomic theory that we know today, along with Joseph Proust’s law of definite composition and the law of conservation of mass proposed by Antoine Lavoisier. These laws paved the way for our current understanding of atomic structure and composition , including concepts such as molecular or chemical formulas .


Some examples of the law already resolved are:

Example 1

The mass percentage of hydrogen in a water molecule is 11.1%, while in hydrogen peroxide it is 5.9%. What is the ratio of hydrogen in each case?

In the water molecule , the hydrogen ratio is equal to O / H = 8/1. In the peroxide molecule it is at O ​​/ H = 16/1. This is explained because the relationship between elements is linked to their mass, so in the case of water there would be a ratio of 16: 2 for each molecule, or what is equal to 8: 1, as illustrated. That is, 16 g of oxygen (one atom) for every 2 g of hydrogen (2 atoms).

Example 2

The nitrogen atom forms five compounds with oxygen that are stable under standard atmospheric conditions (25 ° C, 1 atm). These oxides have the following formulas: N2O, NO, N2O3, N2O4 and N2O5. How can this phenomenon be explained?

By the law of multiple proportions, oxygen is bound to nitrogen with an invariable mass proportion of this (28 g):

  • In N 2 O, the ratio of oxygen (16 g) to nitrogen is approximately 1.
  • In NO, the ratio of oxygen (32 g) to nitrogen is approximately 2.
  • In N 2 O 3 the ratio of oxygen (48 g) to nitrogen is approximately 3.
  • In N 2 O 4 the ratio of oxygen (64 g) to nitrogen is approximately 4.

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