The key difference between law of definite proportions and law of multiple proportions is that the law of definite proportions states the samples of a compound will always contain the same proportion of elements by mass. On the contrary, the law of multiple proportions (sometimes called Dalton’s law) states that if two elements combine to form more than one chemical compound, then the ratios of the masses of the second element that combine with a fixed mass of the first element will be ratios of small whole numbers.
The law of definite proportions and law of multiple proportions are theories used to explain the stoichiometry in chemistry. Stoichiometry is the measure of the relative quantities of reactants and products in a chemical reaction.
CONTENTS
1. Overview and Key Difference
2. What is Law of Definite Proportions
3. What is Law of Multiple Proportions
4. Side by Side Comparison – Law of Definite Proportions vs Law of Multiple Proportions in Tabular Form
5. Summary
What is Law of Definite Proportions?
The law of definite proportions states samples of a compound will always contain the same proportion of elements by mass. In other words, a given compound will always contain the same elements in the same proportions by mass.
For example, either it is tap water or sea water, a water molecule will always contain hydrogen and oxygen elements in the following proportions.
The chemical formula of water molecule = H2O
Molar mass of water molecule = 18 g/mol
Hence, one mole of water contains 18 g of H2O. The ratio between H and O in the water molecule is 2:1. Therefore, the mass fraction of hydrogen in water = (2g / 18g) x 100% = 11.11% and the mass fraction of oxygen = (16g/18g) x 100% = 88.89%. These fractions are definite and do not alter when the source of water and the method of separation.
Figure 01: The Law of Definite Proportions states, in a given type of chemical substance, the elements are always combined in the same proportions by mass.
This law is based on the fact that any atom of the same element (atoms having the same atomic number) is similar to each other. For the above example, it has been considered that any hydrogen atom is similar to that of another hydrogen atom and vice versa. But there are some exceptions as well. For example, isotopic composition of an element can vary based on the source. Hence, the stoichiometry shows variations based on the source of elements.
What is Law of Multiple Proportions?
Law of multiple proportions states when two elements combine with each other to form more than one compound, then the weights of one element that combine with a fixed weight of the other are in a ratio of small whole numbers.
Figure 02: An Explanation of the Law of Multiple Proportions
This is sometimes called Dalton’s law. That was after the discovery of this law by John Dalton in 1803. Let us understand this law using some an example.
- Oxides of nitrogen are composed of nitrogen and oxygen atoms. There are five different oxides of nitrogen are present; N2O, NO, N2O3, NO2 and N2O5. When considered the mass ratios of N and O in these compounds, 14 grams of nitrogen atom combines with 8, 16, 24, 32 and 40 grams of oxygen according to the mass ratio. When taken as small, whole numbers, the ratios are 1:1, 1:2, 1:3, 1:4 and 1:5.
Difference Between Law of Definite Proportions and Law of Multiple Proportions?
The law of definite proportions states samples of a compound will always contain the same proportion of elements by mass. On the other hand, Law of multiple proportions highlights that when two elements combine with each other to form more than one compound, the weights of one element that combine with a fixed weight of the other are in a ratio of small whole numbers. This is the main difference between law of definite proportions and law of multiple proportions.
Summary – Law of Definite Proportions vs Law of Multiple Proportions
The law of definite proportions and law of multiple proportions are used to explain the stoichiometry of compounds in chemical reactions. The law of definite proportions states that the samples of a compound will always contain the same proportion of elements by mass. On the contrary, the law of multiple proportions states that if two elements combine to form more than one chemical compound, then the ratios of the masses of the second element that combine with a fixed mass of the first element will be ratios of small whole numbers. Therefore, this is the difference between law of definite proportions and law of multiple proportions.
Reference:
1. Helmenstine, Anne Marie, Ph.D. “Law of Definite Proportions Definition.” ThoughtCo, Nov. 14, 2017. Available here
2. The Editors of Encyclopædia Britannica. “Law of definite proportions.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 3 Feb. 2017. Available here
3. “Stoichiometry.” Wikipedia, Wikimedia Foundation, 25 Feb. 2018. Available here
Image Courtesy:
1.’Law of Definite Proportions’By CK-12 Foundation (raster), Adrignola (vector) – File:High School Chemistry.pdf, page 141, (Public Domain) via Commons Wikimedia
2.’Demonstration of the Law of Multiple Proportions’By Kenji Agari – Own work, (CC BY-SA 3.0) via Commons Wikimedia