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# Avogadro's Hypothesis - Definition, Examples, practice problems & FAQs

Have you ever gone shopping for fruits at a local fruit vendor’s market? You might have brought different types of fruits depending upon the season of purchase. You might have seen that the vendor would use different kinds of counting systems to measure the quantities like kgs or dozens. While one depends upon weight (kg) while the other depends upon the unit of quantity (dozen).

Depending upon one’s convenience, we may use whichever counting system.

Going along the same flow, even the constituent particles can be quantified using a specific unit.

What is that unit?

Who came up with the idea?

Who established the value of the said unit?

Well, the answer might be surprising.

The quantity is defined as Avogadro’s number. However, the value of Avogadro’s number was not estimated by Avogadro, but the contribution of Avogadro is enormous in the chemical world. Avogadro was the first person who coined the term molecule, he was one of the pioneers in the development of atomic theory and explained the atoms and their form of existence. E.g- A hydrogen atom is H but it can exist in form of H2, an oxygen atom is O but it can exist or form O2 and O3.

• Calculation of vapour density
• Estimation of molar volume of gases
• Molecular formula for gaseous molecules
• Practice problems

⇒ According to Avogadro's Hypothesis, at the same temperature and pressure, the same volume of all gases has an equal number of molecules (not atoms).

The mathematical form of Avogadro's Hypothesis, for ideal gases, (at constant T & P)

where, V = volume, P = pressure, n = number of moles & T = Temperature

• Calculation of vapour density

Vapour density is defined as the ratio of the density of any given gas with respect to hydrogen gas at the same temperature and pressure.

We know, according to Avogadro’s hypothesis, All gases having equal volume have the same number of gaseous molecules (not atoms) at same temperature and pressure.

[at the same Temperature and Pressure, the molar volume of gas = molar volume of H2 gas ]

Molar mass of hydrogen gas = 2 g mol-1

• Estimation of molar volume of gases

Experimentally he measured the molar volume of any gases are same at equal temperature and pressure. E.g- at NTP all gases occupy 22.4 L for 1 mole quantity.

• Molecular formula for gaseous molecules

Gay Lussac’s Law of gaseous volumes

According to Gay Lussac’s law of gaseous volumes, the ratio of the volumes of all gaseous reactants and products can be represented in small whole numbers. When gases react at a particular pressure and temperature, the ratio of volumes of reactants and products can easily be expressed in small whole numbers.

E.g- 1.

One unit volume of hydrogen gas combine with 1 unit volume of chlorine gas to form 2 unit volume of hydrochloric acid.

Note: at that time, the free form of existence of the atom means the difference between atoms and molecules was not known. So, we can’t express equations in H2, Cl2 form. We know, the chemical reaction is a process that involves the rearrangement of bonds and the formation of a new substance takes place, which property is entirely different from the reactant.

According to Dalton, atoms take part in chemical reactions

For 1 hydrochloric acid vapour numerically we need an atom of hydrogen and chlorine gas, according to Dalton's theory, an atom is indivisible. At that time atom and their free form of existence was not known.

Avogadro was the first person who distinguished between an atom and its free form of existence (molecule)

E.g- H is an atom but it exists in form of H2

## Practice problems:

Q1. Equal volumes of gases at the same temperature and pressure, according to Avogadro's theory, will contain:

A. Always the same number of atoms
B. Always same of molecules
C. Same number of atoms and molecules.
D.  None of these

Solution: According to Avogadro's Hypothesis, at the same temperature and pressure, the same volume of all gases has an equal number of molecules (not atoms).

Q2. All gases contain the same number of molecules in equal amounts at the same temperature and pressure. This regulation is referred to as

A. Henry’s law
B. Boyle’s law
C. Law of reciprocal

Solution: According to Avogadro's Hypothesis, at the same temperature and pressure, the same volume of all gases has an equal number of molecules (not atoms).

The mathematical form of Avogadro's Hypothesis, for ideal gases, (at CONSTANT T & P)

Wher, V = volume, P = pressure, n = number of moles & T = Temperature

Q3. At the same temperature and pressure, equal quantities of all gases contain the same number of

A. protons
B. neutrons
C. electrons
D. None of these

Solution: According to Avogadro's Hypothesis, at the same temperature and pressure, the same volume of all gases has an equal number of molecules (not atoms).

In neutral atoms, the number of protons and electrons is always equal. So, in gaseous molecules, all different gases are constituted by different atoms so they always have different numbers of protons and electrons.

Q4. Theoretical explanation of which law was corrected by Avogadro

A. Gay lussac law of gaseous volume
B. Henry Law
C. Law of mass conservation
D. None of these

Solution: Dalton failed to explain the gay lussac law because he was unable to write the molecular formula of homodiatomic molecules like H2, O2 etc. Avogadro coined the term molecules and confirmed that the free form of existence of atoms, an atom of hydrogen is H but it exists in nature in H2 form.

Question 1. What is atomicity?
Answer: Atomicity is the total number of atoms in a molecule or compound.

Atomicity of Cl2 is 2.

Atomicity of H2SO4 is 7 ( 2 hydrogen atoms + 1 sulphur atom + 4 oxygen atoms).

Question 2. What is the importance of Avogadro’s hypothesis?
Answer: Amedeo Avogadro invented this hypothesis in 1811, in which he stated that the volume of gas can be seen independent of either mass or molecular size. Avogadro shows that an equivalent volume of gas at the same temperature and pressure has an equal number of molecules, regardless of its chemical and physical qualities. This law permitted the chemists to forecast the ideal gas performance also this law was found to be very helpful as it acted as a significant way to save a good amount of time and cash in the long run.

Question 3. What were the initial problems faced by Avogadro regarding this hypothesis?
Answer: Scientists found that some of Avogadro’s principles were very difficult to understand because many of its concepts were quite difficult to grasp. Some famous chemists like Gay-Lussac were not quite familiar with these kinds of behaviours of gases and found it difficult to understand and accept Avogadro’s theories.

To address this, Avogadro coined the term 'molecules,' which may be defined as a collection of small particles. He also insisted on its origin, which is: that the word molecule was derived from an old term ‘mole’ which means ‘lumps of matter’. Hence, a molecule can be defined as a cluster of matter.

Question 4. Was Avogadro’s hypothesis applicable only to gaseous molecules?
Answer: Yes, Avogadro’s hypothesis applicable is only applicable to the relation between the number of molecules and volumes of gaseous molecules.

Related Topics

 Strength Molality Mole Parts per million Atomic Mass Definition of Equivalent weight
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