BY: Sharlin Ulfatt
Edited By Pranomita Shom
When dealing with Stoichiometry in Chemistry, one is likely to come across Avogadro's constant at least once. However, have you ever wondered how its first originator, Sir Josef Loschmidt figured it out? He would not have just made one up out of thin air; having some sort of electronic microscopic vision and counting away those nitrogen and oxygen molecules floating away? So let us look at a basic technique to prove it.
One mole of any substance is considered to contain the same number of particles as the number of atoms in 12g (1mol) of Carbon-12 which is approximately 6.02×10^23 particles also known as Avogadro's constant named in honour of the notable Italian scientist Lorenzo Romano Amedeo Carlo Avogadro.
In order to calculate the constant, we will take the example of an Ionic crystal, namely Lithium Fluoride (LiF)
Figure 1.1- Structure of an ionic
crystal of LiF
KEY:
Li+ purple
F- green
Focusing back on the overall ionic crystal, keeping in mind that we are aware of its density and relative molecular mass we can calculate the volume of the crystal:
Density = mass/volume
by subjecting the Volume,
Volume = mass/density
= (25.93g/mol )/(2.64g/cm^3)
= 9.822cm^3
Recalling that the Ionic crystal is a cube, we can calculate its length ( l' ) using its volume.
Volume = l' × l'× l'
= l'^3
by subjecting the length ( l' ),
Length ( l' ) of crystal = (volume)^1/3
= (9.822)^1/3
= 2.14cm
So, using unitary method we can calculate the total number of ions/particles present within one length of the crystal:
As 1 unit cell of ionic crystal length( l ) = 2.01 * 10^-8 cm
Total length of ionic crystal ( l' ) = 2.14cm
Number of ions in one length of crystal=( 2.14 cm)/(2.01 * 10^-8 cm)
= 1.065 * 10^8 ions/ particles
By cubing the number of ions in one length of the crystal, we can find the total number of ions in the entire volume of the crystal:
Total number of ions in the crystal= (1.065 * 10^8)^3
= 1.21 * 10^24 particles/ions
These are the total number of particles present in 2 moles of ions; Li+ and F-
Therefore, the total number of particles in 1 mole of ion:
= (1.21 * 10^24)/2
= 6.02 × 10^23 particles/ions
We have proved that 1 mole of any substance contains 6.02 × 10^23 particles, determining Avogadro's constant.