Dalton’s law

Dalton’s Law . It is one of the most basic stoichiometric laws , formulated in 1803 by John Dalton and includes the law of multiple proportions and the law of partial pressures.


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  • 1 Law of multiple proportions
  • 2 Example
  • 3 Law of partial pressures
  • 4 Formula
  • 5 Applications
  • 6 Sources

Law of multiple proportions

This law states that when two elements combine to originate different compounds , given a fixed quantity of one of them, the different quantities of the other combine with said fixed quantity to produce the compounds, they are in relation to simple integers . This was the last of the weight laws to run. Dalton worked on a phenomenon that Proust had not noticed, and that is the fact that there are some elements that can be related to each other in different proportions to form different compounds. For example, there are two oxides of copper, CuO and Cu2O, which have 79.89% and 88.82% copper, respectively, and which are equivalent to 3,973 grams of copper per gram of oxygen in the first case and 7,945 grams of copper per gram of oxygen In a second. The ratio between both amounts is 1: 2 as currently expressed with the formulas of the compounds derived from atomic theory.


The combination of the same amount of Carbon (12 grams) with different amounts of Oxygen.

C + O2 —- 12 g. of C + 32 g of O2 — 44g.

C + ½ O2 —- 12 g. of C + 16 g of O2 — 28g.

It is observed that the amounts of oxygen maintain the simple numerical relationship (in this case “double”)

32/16 = 2

Law of partial pressures

It states that the pressure of a mixture of gases, which do not react chemically, is equal to the sum of the partial pressures that each of them would exert if only one occupied the entire volume of the mixture, without changing the temperature . Dalton’s law is very useful when we want to determine the relationship between the partial pressures and the total pressure of a mixture of gases. The absolute pressure exerted by a mixture of gases is equal to the sum of the partial pressures of each of the components that make up the mixture. The partial pressure of each gas is the absolute pressure that each component of the mixture would exert separately if it were occupying the entire volume of the mixture.


Pabs = #Ppi

Ppi = Pabs (% ix 100)

Pabs = absolute pressure of a gas

Ppi = Partial pressure of a component of the mixture

  1. Ppi = Sum of the partial pressures of the gases that make up the mixture

% i = Percentage of gas in the mixture


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

PpO2 = 21/100 x 1 = 0.21 atmospheres

PpN2 = 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.

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

PpO2 = 21/100 x 2 = 0.42 atmospheres

PpN2 = 79/100 x 2 = 1.58 atmospheres

PpO2 + PpN2 = 2 ATA

  • The law obliges us that in the mixtures we use, the partial pressure of oxygen cannot exceed 1.4 atmospheres. If we use air (21% O2), what is the maximum allowed depth?

PpO2 = 1.4 ATA

% 02 = 21

Therefore, if we find out at what absolute pressure (Pabs?) Of the air, the PpO2 = 1.4 ATA

Then we will know the depth

Pabs = 1.4 x 100/21 = 6.6 ATA

Depth = (Pabs – 1) x 10 = 56 meters

Max depth it will be 56 meters which is when Pabs = 6.6 ATA


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