Avogadros Hypothesis

Avogadros Hypothesis - Avogadro’s constant


The law of Avogadro provides a means to measure the amount of gas present in a receptacle. This law is sometimes even called as Avogadro's principle or Avogadro's hypothesis. It is a kind of an experimental gas law that relates the gas volume to the quantity of gaseous substance present.

It is a precise case of an ideal gas law. The law of Avogadro states that when all gases of equal volumes are mixed at the same pressure and temperature, then it has the same amounts of molecules.

For any specified weight of a gas, the amount and volume of the gas substance is proportional to each other provided that both the pressure and temperature remains constant.

So, the entire law of Amedeo Avogadro concluded that different types of gases bearing the same volume, when combined at the same pressure and temperature, possess the same amount of in it.

For example, when the two ideal gases, hydrogen and nitrogen are mixed in equal quantities, they contain the same amounts of molecules in them. This happens only when they are kept at the same pressure and temperature. This shows the ideal behavior of gas.

Now let us see this law in mathematical terms:

The entire law can be written as V ∞ n
V / n = K (a constant)


V is the symbol for gas volume;
n is the gaseous substance that is measured in the form of moles;
k is a constant defined for a particular pressure and temperature.
When compared the same gaseous substance under different conditions, this law can be written as:
V1 / n1 = v2/ n2

So, the above equation shows that the quantity of moles in the gas increases in the same proportion as the increase in the gas volume. If the amount of gas moles gets decreased, then its volume also reduces. So, the total number of atoms or molecules present in any particular volume of gas is completely independent of the molar mass or size of the gas.

How the Avagadro’s hypothesis got formed?

Avogadro's hypothesis was expressed in the same way as Boyle's empirical gas law in the year 1662, Charles's law in the year 1787 and Gay-Lussac's law in the year 1808. The Avogadro hypothesis was published by the famous scientist Amadeo Avogadro in the year 1811.
He reconciled the atomic theory of Dalton with an "incompatible" view of other scientists like Gay-Lussac and Joseph Louis that stated a few gases were compound of different types of fundamental molecules in integer proportions.

In the year 1814, Avogadro, and André-Marie Ampère created the same law that resulted in similar outcomes. As the scientist Ampère was more famous in France, this hypothesis got popularly referred to as the Ampère's hypothesis. It later got popular as Avogadro-Ampère hypothesis or Ampère-Avogadro hypothesis.

What problems did Avogadro face before his hypothesis got accepted by people?

Chemists found Avogadro hypothesis quite tough to accept as there were some things that were not easy to accept. One of the things was that if the gaseous particles were combining, then a volume of gas X when reacted with a volume of gas Y should result in one product volume.
Later after several researches, it was found that a volume of gas X when reacted with a volume of gas Y resulted in the formation of two product volumes. Chemists such as Gay-Lussac were not able to interpret this type of gaseous behavior.

Avogadro also reasoned that such an outcome can only be defined when Gay-Lussac’s particles were actually made up of tiny particles. He then introduced the application of the term “molecule” in order to refer to the combinations of small sized particles.

The word “molecule” was derived from the term “mole”. The term “mole” implied “lumps of matter” and the term “cula” implied “little”. So, a molecule was seen as a tiny cluster of matter. The hypothesis given by Avogadro was totally consistent with the assessment that the pressure was caused due to the molecular collisions with the different sides of the container.

On keeping the pressure same, it was pretty reasonable that the amounts of molecules contained in the container also remained the same. Along with it, when hydrogen chloride and ammonia were mixed in equal ratio of one to one, he arrived to the conclusion that a number of different gases mixed in this manner that can only happen if the individual gas molecules were “diatomic”.

Avogadro wasn’t able to base the hypotheses about the diatomic characteristic of molecules. He illustrated several experiments but the scientific community could not accept the validity of the hypothesis till several years post his death.

Using the hypothesis, it assisted the chemists to deduce the chemical formulas of gaseous substances on the basis of combining gas volumes. The application of Avogadro’s hypothesis was instrumental in defining the molar element’s mass.

In what way did Avogadro's hypothesis help chemists?

This hypothesis stated that two different gas samples of equal volume, when present at the same pressure and temperature, contain the equal number of molecules. Avogadro's hypothesis permitted chemists to forecast the performance of an ideal gas.

Amedeo Avogadro created the hypothesis in the year 1811, in which he asserted that the gas volume is independent on the mass or size of the gas molecules. He further stated that equal gas volumes present at the same pressure and temperature contain the equal number of molecules irrespective of their physical properties and chemical nature. This number is referred as Avogadro’s number.

What is Avogadro’s Number?

Avogadro’s number is defined as the number of units in one mole of a substance. This consideration is equal for both the lightest gas (hydrogen) and heavy gases (Bromine or Carbon dioxide). The value of is Avogadro’s Number is 6.022×.

Why is Avogadro hypothesis important?

Avogadro's law established how the gas amount (n) is related to its volume (v). It was found to be a direct relationship, which implied that the gas volume is completely proportional to the amount of gas moles present in it. This law was very significant as it helped in saving a lot of money and time in the long run.

What is the concept of mole and how it was calculated?

Mole is described as the quantity of any substance that is made up of as many molecules, atoms, electrons, ions, or any different forms of objects in 12 grams of carbon. This quantity is referred to as the Avogadro's number.

What is a molar mass?

The mass of one mole of a substance is referred to as its molar mass. This mass is used for the conversion of specific grams of any substance to its moles. It has applications in the field of chemistry too. If one knows the mass of any substance, then the quantity of moles contained in it can be easily calculated.

How many numbers of grams are present in one mole?

There are around 6.022× atoms present in one mole or per twelve grams of carbon. The number of moles present in any substance can be easily found by using the molar mass of that substance. This mass is the number of grams present in a single mole of any substance.