Avogadros Number - Definition, FAQs

Introduction

Avogadro's number or Avogadro constant is the relation factor which relates the number of constituent molecules, atoms or ions in a sample with the volume of substance in that sample. In this article we will answer the questions, what is Avogadro’s number or what is Avogadro’s constant? To study Avogadro number in detail first we have to learn about the term mole which we will study in this article.

This Story also Contains

1. Introduction
2. What is a Mole?
3. Define Avogadro’s Number.
4. Molar Volume
5. How Avogadro’s number (NA) is associated with other constants
6. Few questions related to Avogadro’s number example and moles

What is a Mole?

Mole is defined as the amount of substance that contains the same number of atoms or molecules or particles as there are atoms in 12 grams of carbon-12.

1 mole of carbon-12 = 12 grams of carbon-12 = 6.022 ×10²³ atoms

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One mole of atoms = 6.022 ×10²³ atoms

One mole of molecules = 6.022 ×10²³ molecules

In other words,

Number of particles in 1 mole = number of atoms in 1 mole of carbon-12

Where, Carbon-12 is one of the isotope of carbon and is used as a standard unit for Mole.

Define Avogadro’s Number.

Ever wondered how many atoms are in the universe or in your body? How can you count a quantity as small as an atom?

To answer these questions, in 1811 Italian chemist Avogadro, full name of Avogadro is Lorenzo Romano Amedeo Carlo Avogadro, was thinking of the atoms and molecules inside a flask from those ideas he came up with the thought of Avogadro's hypothesis which specified that, "if the physical conditions are same that is temperature, pressure and volume then the molecules presents will also be same”.

Let’s take an example to understand this statement further.

We take a container whose capacity is 100 ml and if we put hydrogen gas in it and we measure the number of molecules and say the number of molecules comes out to be one lakh. Now, let’s take another same container whose capacity is also 100 ml and if we pot oxygen gas inside it. If the condition are same that is temperature, pressure and other physical quantities are same. So, the number of molecules in oxygen gas will also be one lakh only under similar conditions of temperature and pressure.

But most of the scientists at that time look at Avogadro's work as a hypothesis and didn't gave it much thought to it. In 1860, his work was proven correct but unfortunately Avogadro died in 1856.This hypothesis made it easier to perform calculations involving gases, chemist use it in the same way as we use grams to buy grapes, vegetables or meat. This means that 1 mole of any gas will occupy the same volume as 1 mole of any other gas provided that they are at the same temperature and pressure.

So what is Avogadro number? The Avogadro number is 6.022 ×10²³ also known as Avogadro's constant or NA. There is no difference between Avogadro number and Avogadro constant, they are one or the same thing. This is the same number of particles as there are in exactly 12 g of cabon-12. If we try to count Avogadro’s number and if 10000 people start to count Avogadro’s number and counted at the rate of 100 numbers per minute each minute of the day, it would take over 1 trillion years to count the total number and if one mole of marbles were spread over the surface of the earth, our planet would be covered by a 20 mile dense layer of marbles.

Avogadro's number unit is mole inverse (mole-1)

Detail explanation of Avogadro's number.

Avogadro number is 6.022 ×10²³ just a number to represent quantities. It is similar to how we use the word 1 pair to represent 2, 1 dozen to represent 12, 1 baker’s dozen to represent 13, 1 gross to represent 144 and 1 Ream to represent 500.Similarly, Avogadro number is the term to represent 6.022 ×10²³ number of any entity. Avogadro's number is not used by us in daily life and not advisable to use. So why does we use the number exists if we do not use it in daily life? Yes, the number is not used in everyday life though it approaches to the rescue of chemists that works with atoms, molecules, electrons and many more such small entities. That is the reason why it is used to calculate 1 mole of any substance.

1 mole of carbon-12 = 12 grams of carbon-12 = 6.022 ×10²³ atoms

What will be the 1 mole of pure gold?

1 mole of pure gold = 196 grams of pure gold = 6.022 ×10²³ atoms

Similarly,

1 mole of pure water = 18 grams of pure water = 6.022 ×10²³ molecules

But there is a point to be noted, if we take one mole of gold, one mole of silver and one mole of copper.

They will have same number of atoms that is 6.022×10²³ atoms but the mass of each one will be different because the mass of individual atom which make these element are different.

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Molar Volume

Molar volume is defined as the volume occupied by one mole of gas. Molar volume is measured in dm³mol¹.

At room temperature and pressure the molar volume is taken as 24 dm³mole-1.

Let's solve an example to get it better.

Find the volume occupied by 8 grams of O₂ at room temp?

1 mole = 24dm³

And,

1 mole of O₂ = 32g

32 g of O₂= 24 dm³

1g = 24dm³/32

So,

4g= 4×24 dm³/32

=3dm³

How Avogadro’s number (NA) is associated with other constants

The Avogadro’s number is associated with other physical constants and properties in following ways:

• Relation between R which is molar gas constant, NA which is Avogadro’s number and Kb which is Boltzmann’s constant is

R = Kb.NA

Where, Kb=1.38 X 10^-23J/K and NA=6.022×10²³

=8.3144 J.K.mol

• Relation between faraday constant (F) , Avogadro’s number (NA) and elementary charge (e) is

F = e.NA

Where, e= 1.6 x 10^-19 C and NA= 6.022×10²³

=96485.3321 C/mol

• Relation between molar mass constant (Mu), Avogadro’s number (NA) and atomic mass constant (mu) is

Mu=mu.NA

Where, mu= 1.66 x10^-27 Cmol and NA=6.022×10²³

=0.9999999 x10^-3 kg. mol-1

Few questions related to Avogadro’s number example and moles

1. Find number of moles of atoms in 120 g of calcium. Also find the number of atoms present.{Ca =40}

Number of moles = mass given/molar mass = 120/40

=3 moles of calcium atoms

For the number of atoms present,

One mole = 6.022 ×10²³ atoms

=3 x 6.022 x 10²³ calcium atoms

NCERT Physics Notes :

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2. Find number of moles of molecules in 34 g of NH₃. And number of molecules present. [N=14,H=1]

Number of moles = mass given/molar mass = 34/17

= 2 moles of NH₃ molecule

For the number of molecules present,

One mole = 6.022 ×10²³ molecules

=2x 6.022 x 10²³ NH₃ molecules

3. Find the atoms which are present in 42.67 mole of copper?

One mole = 6.022 ×10²³ molecules

For the number of atoms present in 42.67 mole of copper,

=42.67 x 6.022 x 10²³ atoms

=2.570 x 10²⁵ atoms

4. Find the molecules which are present in 7.32 moles of sulphur dioxide?

One mole = 6.022 ×10²³ molecules

For the number of molecules present in 7.32 mole of sulphur dioxide,

=7.32 x 6.022 x 10²³ molecules

=4.41 x 10²⁴ molecules

Also check-

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