[Chemistry Class Notes] Law of Multiple Proportions Pdf for Exam

In chemistry, the law of multiple proportions can be defined as if two elements form more than one compound between them, the mass ratios of the second element that combine with a fixed mass of the first element will always be the ratios of small whole numbers. Sometimes, this law is referred to as Dalton’s Law (or Dalton’s Law of multiple proportions) because it is named after John Dalton, the chemist who expressed it first.

The Law of Multiple Proportions is defined as:  “if two elements combine to form more than one compound, the mass ratios of the second element that combine with a fixed mass of the first element will always be ratios of minuscule whole numbers.” This law, sometimes known as Dalton’s Law or Dalton’s Law of Multiple Proportions, was proposed by Dalton in 1803.

Hydrogen, for example, reacts with oxygen to generate two compounds: water and hydrogen peroxide.

Hydrogen + Oxygen → Water 

2g 16g 18g

Hydrogen + Oxygen → Hydrogen Peroxide

2g 32g 34g

 

Examples of Law of Multiple Proportions

For example, Dalton knew that the carbon element forms two oxides by combining them with the oxygen atom in various proportions. A fixed mass of carbon compound, let us suppose 100 grams, can react with 266 grams of oxygen to form one oxide atom or with 133 grams of oxygen to form the other. The ratio of the oxygen masses that can react with the 100 grams of carbon is given as 266:133 = 2:1, which is a ratio of small whole numbers. Dalton has interpreted these results in his atomic theory by proposing that the two oxides have one oxygen atom and two oxygen atoms, respectively, for every carbon atom. In modern notation, the first is given as CO (carbon monoxide), and the second is given as [CO_{2}] (carbon dioxide).

First, John Dalton expressed this specific observation in 1804. But, a few years before, the French chemist named Joseph Proust had proposed the law of definite proportions, which has expressed that the elements combined to produce compounds in certain well-defined proportions, instead of mixing in just any proportion. And, Antoine Lavoisier has proved the law of conservation of mass that helped out Dalton. A careful study of the exact numerical values of these proportions has led Dalton to propose his law of multiple proportions. This was an essential step toward the atomic theory that he would propose later the same year, and it laid the basis for the compound’s chemical formulas.

Another example of the law is seen by comparing the ethane ([C_{2}H_{6}]) compound with the propane ([C_{3}H_{8}]) compound. The weight of hydrogen that combines with 1 gram carbon is 0.252 gram in the ethane and 0.224 gram in the propane. The ratio of those weights is 1.125, which is equal to the ratio of two small numbers (9:8).

Dalton’s Atomic Theory

Democritus has first suggested the atom’s existence. However, it took almost two centuries for the atom to be established as a basic chemical entity, which was accomplished by John Dalton (from the years 1766-1844). Dalton’s atomic theory remains valid in modern chemical thought, although two centuries old.

Important Points of Dalton’s Atomic Theory

• All atoms of the given element are identical in properties and mass

• All matter is completely made of atoms, and these atoms are indestructible and indivisible

• A chemical reaction is given as a rearrangement of atoms

• Compounds are produced by the combination of either two or more various kinds of atoms

Modern atomic theory is given as a little more involved than Dalton’s theory. But, the essence of Dalton’s theory remains completely valid. Currently, we know that atoms are destroyed by nuclear reactions but not by chemical reactions. Also, there are various kinds of atoms (differ by their masses) within an element that are called “isotopes”, but the isotopes of an element contain similar chemical properties.

Several heretofore unexplained chemical methods were quickly explained by Dalton with his specific theory. In chemistry, Dalton’s theory very quickly became the theoretical foundation.

To put it simply and briefly, the atomic structure of a compound is a vital component to understanding the behavior and reactivity of chemicals. Simply by looking at the structure of a molecule, a chemist can be able to predict several things about a chemical such as :

• Boiling points and melting points

• Acidity or basicity

• Reactivity with the other compounds

• What sort of products that two chemicals might produce in a chemical reaction

The list goes on. Not only are there easy and simple ways to predict the properties of compounds, but there are also things that wouldn’t be obvious immediately. As an example, several molecules will contain two forms, which are mirror images of each other (which can be based on the arrangement where the molecules are bonded). Every mirror image contains drastically different effects, like in Thalidomide. One mirror image of this molecule can be used as a useful drug, but also the other image, as it turned out, was a carcinogen.

Basically, both the short and long of it is that if we understand the atomic structure of a compound, we can rationalize the effects that we didn’t necessarily predict and predict its chemical behavior.

Conclusion

The law of multiple proportions is a fundamental rule of chemistry. According to the law of multiple proportions, if two elements can react to produce several compounds, the ratio of their masses that mix with a fixed mass of the other element is in the ratio of tiny whole numbers. It was instrumental in the early understanding of stoichiometry, along with the law of definite proportions. The concept was proposed in 1803, by English chemist John Dalton, who is best known for his partial pressures law. Dalton stated the law in his book New System of Chemical Philosophy (Vol 1).

Proust’s law of definite proportions was followed by the law of multiple proportions. The law backed up Proust’s work and later aided Dalton in his atomic theory.