Dalton's law

What is Dalton's law?

Dalton's law or the law of multiple proportions can be stated as follows:

When two elements combine to form more than one compound, a constant mass of one of them combines with variable masses of the other. These variable masses have a simple integer relationship to each other.

In 1803, the English chemist John Dalton (1766-1844) formulated the law that bears his name. It calculates the total pressure from the partial pressures of a mixture of gases, and the multiple proportions of an element. Thus allowing the interpretation of the weight laws of chemical combinations and the understanding of the transformations of one substance into others.

Dalton's experiment

Dalton states that, at a stable temperature, the total pressure of a mixture of gases that do not react chemically with each other, is equal to the sum of the partial pressures that would exert each of its components, as if it were only one that occupies the entire volume of the mixture.

Since the pressure of an ideal gas mixture depends only on the number of gas molecules in the container, Dalton's law is very useful when we want to determine the total pressure of a mixture. Dalton's law is very useful when we want to determine the total pressure of a mixture.

It is based on the combination of an element with the amount of another element to form a different compound, as long as they have a relationship of whole numbers. When we combine two elements, we can give rise to two or more chemical compounds, the amounts of one of them, and the fixed amount of the other keeping in this way, a relationship of simple integers. In this way Dalton tried to unify the symbols, so that with a single representation it would be understood which element was involved.

Explanation of Dalton's law

This law arose in 1803. It explains that quantities of the same element, which are joined with a fixed quantity of another element, form a different compound in each case, being in a ratio of simple integers.

Dalton observed and studied a phenomenon that Proust had not described, in which some elements with each other in different proportions originate different compounds. Today it is known as the different oxidation states of an element, which allows it to combine in different proportions with another element.

Dalton discovered that a water molecule (H2O) always contains the same proportion of oxygen (O) and hydrogen (H). He thought that oxygen atoms were heavier than hydrogen atoms and so it is, the oxygen atom weighs 16 times more than the hydrogen atom.

Therefore, the law of multiple proportions became a key proof for Dalton's atomic theory, although today there are doubts as to whether it was based on this law or whether he discovered it by accident.

Example of Dalton's law

It should be remembered that, when this law was formulated, certain verifications could not be carried out, since they were of an empirical nature.

Thus we take copper and oxygen, combine to form two distinct copper oxides, copper (II) oxide CuO and copper (I) oxide Cu2O.

In CuO, 3.97 grams of copper combine with 1 gram of oxygen (63,546/15,999). In the second compound Cu2O, 7.94 grams of copper binds to each gram of oxygen (2 x 63,546/15,999).

We look for the ratio in 7.94/3.97 and get a simple integer, 2 as predicted by Dalton.