Law of Definite Proportions

Law of Definite Proportions Definition

Let's break down what the law of definite proportions definition actually means!

We are going to look at all the bolded aspects of the law of definite proportion definition:

• Remember, compounds are a class of matter known as pure substances, so the elemental composition of compounds is bonded atoms from two or more elements
• according to this law, every sample of a particular compound has a fixed ratio of elements meaning the same elements in the same proportions by mass
• We are looking at the ratio of elements to compounds according to their molecular weights, which is the percent composition.

The implication of the law of definite proportion is that no matter the source of the elements or the amount of the compound, the ratio of elements must be constant to form the compound.

Fig.1-According to the law of definite proportions, pure water (H₂O) from any source has the same percentage composition of Hydrogen and Oxygen.

Law of Definite Proportions Formula

Now that we are familiar with the conceptual side of the law of definite proportions let's take a look at a formula that shows the relationship between compounds and fixed ratios!

An easy way to represent the law of definite proportions is through the percent composition formula :$$\begin {align}\%\,Composition\,of\,AB &= \\\%\,of\,A &= \frac{mass\,of\,element\,A*atoms\,of\,element\,A}{total\,molecular\,mass\,of\,AB}*100\% \\\%\,of\,B &= \frac{mass\,of\,element\,B*atoms\,of\,element\,B}{total\,molecular\,mass\,of\,AB}*100\% \\\end {align}$$

Next, we will do a couple of examples applying the law of definite proportion and determine the percent composition of various compounds!

Law of Definite Proportions Examples

A broad example of the law of definite proportions would be Glucose (C₆H₁₂O₆). Glucose is always made up of 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. This is the only ratio of these three elements that can make glucose.

Keeping this in mind, we will go over a thorough example of applying the law of definite proportion by calculating the percent composition of glucose together.

Here is a different type of question applying the law of definite proportions!

Law of Definite Proportions Experiment

Maybe you think that the law of definite proportion seems pretty straightforward. Of course, a compound would have to be made up of the same ratio of elements every time! But, just like the law of gravity wasn't always obvious, the concept of definite proportions in compounds was once unknown!

In the early 1800s, a chemist named Joseph Proust experimented with copper carbonate (CH₂Cu₂O₅). Proust synthesized artificial copper carbonate and compared this lab product to naturally occurring copper carbonate. After analyzing both substances, Provost found that both samples of copper carbonate contained the same weight proportions of the three elements involved: carbon, oxygen and copper. In Proust's own words, the experiment showed that

Thus, the law of definite proportions was born!

Law of Definite Proportions vs. Law of Multiple Proportions

Even though Proust published his law around 1794, it wasn't until chemist John Dalton published his atomic theory and his law of multiple proportions around 1804 that the law of definite proportions became widely accepted.

The law of multiple proportions would explain compounds such as:

• NO, NO₂
• CO, CO₂
• CH₂, CH₄

These compounds are made up of the same elements, but have different ratios of the second element in the two compounds.

Fig.2-NO and NO₂ proportions