What is Avogadro’s Number? Revision notes for CBSE Class 11 Physics

The Avogadro’s constant is being used to find the number of particles contained in 1 mole of a substance. The nature of the particles can be anything such as atoms or molecules, or electrons. It is a fundamental topic in Class 11 Physics, and the more the students know about it, the easier it will be to score in the examination.

In this educational article, we will be having a wider look at this number while discussing all the related concepts in detail so that it becomes simpler for the students to obtain the best possible marks.

Avogadro’s number – Definition

In CBSE Class 11, Avogadro’s number is defined as the number of particles present in 1 mole of any specific substance. Mole can also be written as mol. These particles can either be molecules, electrons, or atoms. Moreover, the nature of the particles depends on the nature of the substance and the character of the reaction. The approximate value of Avogadro’s constant is 6.022*1023 mol-1.

For example, to calculate the number of particles present in 5 moles of a substance, the following method needs to be used:

=5*6.022*1023 

=3.011*1024 particles

Importance of Avogadro’s constant

In Class 11 Physics, the importance of Avogadro’s constant is also discussed. In simple words, it can be seen as a bridge between the macroscopic world and the microscopic world. This connection can be obtained by relating the number of particles to the amount of substance. This constant can also provide the relationship between other properties and physical constraints. The following are some of them:

• The relationship between the atomic mass unit (u) and the molar mass constant Mu:1 u=MuNa.
• The relationship between the constant of Faraday (F), and the elementary charge, e: F=NA e.
• The relationship between the gas constant and the Boltzmann constant, kB=R=kg NA.

In terms of chemistry, usually, the approach we take is macroscopic to measure things. It is possible to measure things like the temperature of the substance. Furthermore, we also measure the volume and mass of the substance. While looking at it from an atomic level, knowledge of velocity and the particles’ moment is important. More importantly, atomic mass is essential. Avogadro number acts as a connection between these two.

How to determine Avogadro’s number?

The time when Avogadro grew up was when there were huge innovations in the world of Science, especially in the chemistry area. At first, the basic properties of molecules and atoms were understood by Joseph Gay-Lussac and John Dalton. They even debated how these tiny particles behaved. Furthermore, a work of Gay-Lussac named Gay-Lussac’s law, impressed Avogadro. It included a combination of volumes.

Using this law, Avogadro made a few changes and declared the goal for this to be valid. He identified that an equivalent number of particles could be obtained from equivalent volumes of two different gases at similar weight and temperature. To confirm whether this law is true, identifying the difference between molecules, atoms, and some other elements is the only way to do it. For example, N2 is the short form of Nitrogen that exists as a molecule.

Moreover, a disadvantage of Avogadro is that it does not explain molecules in theory, and the ideas were found the opposite to that of John Dalton, among others. However, another chemist named Stanislao Cannizzaro brought the ideas of Avogadro that it deserved. Unfortunately, Avogadro died during the time his law became famous.

Avogadro’s law was the essential key to the advancement of Chemistry. Due to this reason, the name Avogadro’s number was given by another renowned chemist named Jean Baptiste Perrin in his honour.

Significance of Avogadro’s number

The CBSE Physics students are taught the significance of Avogadro’s number. The following are some of its significance:

The substances are measured according to the atomic mass unit at the atomic level. The atomic mass unit can be determined as the 112th of the mass weight of a single carbon atom. 

For instance, the atomic mass unit of hydrogen can be given as 1.00794 amu. To calculate the single particle’s ability, it is impossible to carry out a reaction. These particles can be atoms, molecules or electrons.

Due to this reason, the chemists figured out a way to link the gram and the atomic mass unit:

1 amu=1.66*10-24 gram

With its help, it is possible to convert the measurement of the atomic mass unit to the measurement in grams and vice versa.

Mole concept

While dealing with Avogadro’s number, it is inevitable to talk about the concepts of moles. The mole concept is a convenient technique that helps express the amount present in a substance. It is easy to break all types of measurements into two main parts; the numerical magnitude and the unit in which it is expressed. 

For example, if we measure the box containing 10 kilograms, the ’10’ is considered the magnitude while ‘kilograms’ is its unit. 

In addition to this, even a gram of a pure element can contain an enormous amount of atoms in atomic or molecular particles. Because of this, the usage of the mole concept is tremendous. Therefore, to define a mole, it is nothing but the primary focus of a unit. It can also be regarded as a large count of particles.

Mole – Explanation

In Class 11 physics, a mole can be determined as a substance that contains exactly 6.022*1023 elementary entities of the substance. This number called Avogadro’s constant and is expressed as ‘NA‘. The elementary entities that could represent moles are monatomic ions, polyatomic ions, molecules, atoms, and other related particles, especially electrons. 

For example, a pure carbon 12 sample (C – 12) contains a mass of 12 grams and has a 6.022*1023 (NA) number of C – 12 atoms. The given below is the formula that could specifically state the number of moles of a substance present in a provided pure sample.

n=NNA

where,

n represents the number of moles present in the substance or elementary entity

N – total number of elementary entities in the sample

NA is the Avogadro’s constant

Wilhelm Ostwald, a German chemist, invented the word ‘mole’ in 1896. It was derived from a Latin word, moles, which means ‘heap’ or ‘pile’.

Usually, the number of moles of a molecule need not necessarily be equal to the number of moles of its constituent elements. For instance, a mole of water contains NA number of H2 O molecules. In addition to this, all the water molecules are contained with two hydrogen atoms and one oxygen atom. So, it illustrates that one mole of H2 O has two moles of hydrogen and one mole of oxygen.

Related formulae

In Class 11, for calculating the number of moles in a provided sample of a compound or an element, one could divide the total mass of the sample by the molar mass of the element. Given below is the formula for that,

No. of atoms or molecules= Mass of the sample Molar mass

While calculating the total number of atoms or molecules in a sample, we should multiply the number of moles with the Avogadro constant. Provided below is the formula for the same,

No. of atoms or molecules=No. of moles*6.022*1023

To find out the relation between the atomic mass unit (amu) and the gram of the substance, the following formula is used,

1 amu= 1 gram6.022*1023=1.66*10–24 grams

Hence, the mass of one mole of an element is equal to its atomic mass in grams. 

Applications of Avogadro’s law

The applications of Avogadro’s law are tremendous. The following are some of the areas where this law can be used:

• This law plays a vital role in explaining Gay Lussac’s law of combining volumes.
• Avogadro’s law help in determining the atomicity of gases such as N2, H2 and Cl2, etc.
• This law is used in relating mass and vapour density.
• Apart from that, it also is useful in relating the gas’s gram molar volume and gram molecular mass (GMM) at STP.
• Avogadro’s law assists in determining the gas’s molecular formula.

If you are a student of Class 11 and preparing for your exams, then these pages are certainly going to help you:

NCERT Solutions for class 11

NCERT Solutions for Class 11 Physics

Conclusion

In conclusion, the students of CBSE Class 11 are taught so many varieties of topics under the concept called Avogadro’s number. At first, they are educated on what that number is and how it can be used on different occasions. Later on, the students learned its importance and significance while also understanding how to determine the constant.

In addition to Avogadro’s number, the students are deeply taught on a topic called mole concepts, where they learnt everything about moles and their explanation. Apart from that, all the other related formulae are also discussed. Finally, applications of Avogadro’s law were taught to the students of Class 11 Physics. All these inputs will play a key role in helping the students succeed in their examinations, especially in this particular topic.

FAQs

1. What are the three signs of the mole?

The following are the three significant capabilities of a mole,

• • The mole concept has the capability of providing us with a universally approved standard for measuring masses.
  • The mole concept assists in giving a detailed explanation of the properties of water.

• The mole concept also plays an essential role in helping us establish a standard for the stoichiometry of reactions.

2. What is the main purpose of mole unit fraction?

A mole unit fraction is defined as the number of single component molecules (or moles) which is divided by the total number of molecules (or moles) in a specified mixture. While mixing two reactive components, a mole fraction could be found useful. It is because the ratio of two components is easily understandable if a mole fraction is known for each component.

3. What is the specialty of Avogadro’s law?

Avogadro’s number is considered one of the most essential constants in the field of Science. It allows us to differentiate between various atoms and molecules of a provided substance where the same and an equal number of molecules is getting compared.

4. Why is Avogadro’s number constant?

The choosing of the Avogadro’s constant was due to the mass contained in one mole of the chemical compound. It is usually in grams and is numerically equal to the one molecule’s average mass, which is noted in daltons. It can be utilized for all practical purposes. One dalton is 112 of one carbon – 12 atom’s mass.

5. Who calculated Avogadro’s constant?

A German scientist named Johann Josef Loschmidt was responsible for the calculation of this constant in the year 1865. He was also the person who calculated the Loschmidt number. This number can be used as a constant capable of measuring the same thing as the Avogadro’s constant. However, it will be in different units.