Basic Concepts of Mole, Avogadro’s Number and Molar Volume

Mole- A chemist’s secret unit


A mole is specified as the quantity(mass) of a compound that contains 6.02 ×l023 number of particles (atoms, particles, or formula units). It establishes a link between the mass of a compound and the number of particles of molar computations.

Representation: It is abbreviated as ‘mol’.


We know that atom is an exceptionally little particle. The mass of an individual atom is an extremely small quantity. It is not possible to weigh individual atoms or even a small number of atoms directly. That is why we use the atomic mass unit(AMU) to express the atomic masses. For the sake of benefit, the atomic mass might be given up any unit of measurement i.e. grams, kg, pounds, and so on.

When the compound at our disposal is an element then the atomic mass of that component expressed in grams is called one gram atom. It is likewise called one-gram mole or just a mole of that element.

Number of gram atoms= Mass of an element in grams/

 or moles of a componentMolar mass of a component

For example

1-gram atom of hydrogen = 1.008 g.

1-gram atom of carbon = 12.000 g.

and 1-gram atom of uranium = 238.0 g

It suggests that one gram atom of different components have different masses in them. One mole of carbon is 12 g, while 1 mole of magnesium is 24g. It likewise shows that an individual atom of magnesium is two times as heavy as an atom of carbon. The molecular mass of a compound expressed in grams is called gram molecule or gram mole or simply the mole of a substance.

Variety of gram particles or moles of a molecular substance = Mass of molecular substance in grams/ Molar mass of the substance.

For instance.

1gram molecule of water = 18.0 g.

1gram particle of H2SO4 = 98.0 g.

and 1gram molecule of sucrose = 342.0 g

It indicates that one gram molecule of different molecular compounds has different masses. The formula unit mass of an ionic substance expressed in grams is called the gram formula of the compound. Given that ionic compounds do not exist in molecular type therefore the sum of atomic masses of individual ions gives the formula mass. The gram formula is likewise described as gram mole or simply a mole.

Further Reading:  Functional Groups

Number of gram formulas or moles of a substance = Mass of the ionic substance in grams/ Solution mass of the ionic substance

1gram formula of NaCl = 58.50 g.

1gram formula of Na2CO3 = 106 g.

1gram formula of AgNO3 = 170g.

It might also be discussed here that the ionic mass of an ionic type expressed in grams is called one gram ion or one mole of ions.

For instance:

1 g ion of OH- = 17g.

1 g ion of SO42-= 96g.

1 g ion of CO3 2- =60g.

So, the atomic mass, molecular mass, formula mass, or ionic mass of the compound expressed in gram is called the molar mass of the substance.

Therefore, the quantitative meaning of mole is the atomic mass, molecular mass, or formula mass of a compound expressed in grams is called a mole.

Avogadro’s Number


Avogadro’s number is the number of atoms, molecules, and ions in one gram atom of an element, one gram molecule of a compound, and one gram ion of a substance, respectively.

Representation: It is represented by the symbol ‘NA ‘.


It is interesting to know that different masses of elements have the same number of atoms. Hence, the 6.02 ×l023number of atoms, molecules, or A formula units is called Avogadro’s number that is equivalent to one ‘mole’ of the respective compound. In basic words, 6.02 ×l023particles amount to one mole as twelve eggs are equal to one dozen.

To understand the relationship between Avogadro’s number and the mole of a substance let us consider a few examples.

  • i. 6.02 ×l023atoms of carbon are equivalent to one mole of carbon.
  • ii. 6.02 ×l023molecules of water are equivalent to one mole of water.
  • iii. 6.02 ×l023formula units of NaCl are equivalent to one mole of sodium chloride.
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Thus, 6.02 ×l023atoms of elements or 6.02 ×l023particles of molecular substance or 6.02 ×l023units of ionic substances are equivalent to 1 mole.

For further explanation about the number of atoms in molecular substances or the number of ions in ionic substances let us discuss two examples:

  1. One particle of water is comprised of 2 atoms of hydrogen and 1 atom of oxygen, hence 2×6.02 ×l023atoms of hydrogen and 6.02 ×l023atoms of oxygen make up one mole of water.
  2. One formula unit of sodium chloride includes one sodium ion and one chloride ion. So, there are 6.02 ×l023variety of Na+ ions and 6.02 ×l023CI ions in one mole of sodium chloride. Therefore, the overall number of ions in 1 mole of NaCl is 12.04× l023 or 1.204× 1024.
Molar Volume

One mole of any gas at standard temperature and pressure (STP) occupies a volume of 22.414 dm3. This volume of 22.414 dm3 is called molar volume and it holds true only when the gas is ideal.

Ideal gas: An ideal gas is defined as one in which all collisions in between atoms or molecules are perfectly elastic and in which there are no intermolecular attractive forces. One can say it as a collection of completely hard spheres which collide but which otherwise do not interact with each other.

With the help of this information, we can convert the mass of a gas at STP into its volume and vice versa.

For this reason, we can state that.

  • 2.016 g of H2 = 1 mole of H2 = 6.02 x 1023 molecules of H2 = 22.414 dm3 of H2 at S.T.P.
  • 16g of CH4 = 1 mole of CH4 = 6.02 x 1023 particles of CH4= 22.414 dm3 of CH4 at S.T.P.

It is very intriguing to understand from the above information that 22.414 dm3 of each gas has a different mass however the very same number of particles. The reason is that the masses and the sizes of the molecules don’t affect the volumes. Generally, it is understood that in the gaseous state the distance between molecules is 300 times greater than their diameters.