Proust's Law of Definite Proportions — Overview & Origin

Proust's Law of Definite Proportions

• Proust's Law of Definite Proportions was created by Joseph Proust (a French chemist) in 1794 after realizing during his experiments that copper (II) carbonate is always in a fixed mass ratio of 5.3:1:4 (copper:carbon:oxygen)

• Originally scientists believed that compounds could be composed of different mass ratios of elements and still be the same compound. This was disproved through Proust's research which is why this law exists today!

What is Proust’s Law of Definite Proportions?

Note: Proust's Law of Definite Proportions is based off mass. A compound always contains the same proportion (by mass) of elements

Visualizing Proust's Law of Definite Proportions

• Remember, a compound contain a fixed ratio of elements, if that ratio changes, then that is a new unique compound

Example: A compound with 43% Sodium and 57% Chlorine is different than a compound containing 50% Sodium and 50% Chlorine

The Law of Definite Proportions, proposed by Joseph Proust, states that a compound is always composed of the same elements by mass. In other words, in a compound, the ratios of the elements included are always the same, no matter the total amount of the compound. A different ratio of elements would result in an entirely different compound.

For example, 31 grams of water includes 3.469 grams of hydrogen and 27.53 grams of oxygen. This means there is 11.19% of hydrogen and 88.81% of oxygen included. Now take just 8 grams of water, and there is .8951 grams of hydrogen and 7.105 grams of oxygen. If you do the math, the percentages are equal as the 31 grams of water.

Although today, this law may seem a bit obvious, at the time, some scientists believed that compounds could be made with any amount of the elements needed. Proust proved this by performing experiments that showed copper carbonate always contained the same proportions of carbon, oxygen, and copper, no matter what. This law, along with the Law of Multiple Proportions, formed the basis for stoichiometry. We must note, however, that this law is not always true, and there are some exceptions.

(Video) Chemistry 5.8a Law of Definite Proportions

by IsaacsTEACH

In this video, you will learn why compounds are always formed from a fixed ratio of elements. The ratio will always be the same no matter how big or small of a sample you have. This also applies to the compound's mass ratio.

Water is made of the elements hydrogen and oxygen- but is any mix of hydrogen and oxygen water? Even without a background in chemistry, it feels pretty instinctive that it isn't, but early chemists weren't certain.

Joseph Proust was the first to show that every substance contains a specific mass ratio of elements to each other that never changes. In other words, if you were to add more hydrogen atoms to any amount of water molecules, the hydrogen wouldn't form more water, because there need to be 2 hydrogens per oxygen.

Hydrogen atoms have an average mass of 1.01 g/mol, while oxygen atoms have an average mass of 16.00 g/mol. This means that out of 18.02 g of water, 2.02 g have to be hydrogen, while 16.00 are oxygen. Regardless of the amount of water present, hydrogen and oxygen will still be there in the same mass ratio: 2.02 out of every 18.02 grams, or 11.2% by mass will be hydrogen, while 88.8% by mass will be oxygen.

This is Proust's Law of Definite Proportion: mass proportions of any substance will always stay the same, regardless of how much of the substance there is. Along with Dalton's Law of Multiple Proportions, this forms the basis of stoichiometry, or the study of the ratios in which elements combine into chemical compounds.

The Law of Definite Proportions states that the mass ratios of elements are always the same for the same compound. Let's take carbon dioxide as an example. In carbon dioxide, $27.3\%$ of the mass is is carbon, and $72.7\%$ is oxygen. This means that if we had $15$ grams of carbon dioxide, it would be composed of $4.09$ grams of carbon and $10.91$ grams of oxygen.

Most compounds have different mass ratios. This is because most compounds have different chemical formulas. Proust's law directly supports the idea that a compound has a specified chemical formula that doesn't change. If it did change then the mass ratios would not always be the same for the same compound. Let's think about carbon dioxide again. Carbon dioxide has the formula $CO_{2}$, and carbon has a mass of $12.01$ AMU, and oxygen a mass of $16.00$ AMU. Put together, $CO_{2}$ has a mass of $44.01$ AMU. If we find the ratio of carbon over the total mass, we see that in each molecule, carbon accounts for $27.3\%$ of the mass. In the same process we can see that oxygen accounts for $72.7\%$ of the mass. These strict chemical formulas are the underlying reason for the law of definite proportions.