What is Avogadro’s number? Definition, Units, Examples

What is Avogadro’s number?

One mole of a substance is equal to 6.022 × 10²³ units of that substance (such as atoms, molecules, or ions). The number 6.022 × 10²³ is known as Avogadro’s number or Avogadro’s constant.

It is identical to 6.02×1023. The particles can be electrons, ions, atoms, or molecules, relying upon the man or woman of the response and the character of the substance.

To understand Avogadro’s number let us consider some examples.

1.008 g of hydrogen = 1 mole of H = 6.02×10²³ atoms of hydrogen

23 g of sodium = 1 mole of Na = 6.02×10²³ atoms of Na

238 g of uranium = 1 mole of U = 6.02×10²³ atoms of U

This number, 6.02×10²³is the number of atoms in one mole of the element. It is interesting to know that different masses of elements have the same number of atoms. An atom of sodium is 23 times heavier than an atom of hydrogen. In order in order to have an equal number of atoms sodium should be taken 23 times greater in mass than hydrogen. Magnesium atom is twice heavier as carbon. I.e. 10g of Mg and 5g of C contain the same number of atoms.

18 g of H₂O = 1 mol of water  =   6.02×10²³ molecule of water

180 g of glucose  = 1 mole of glucose = 6.02×10²³ molecule of glucose

342 g of sucrose = 1 mole of sucrose = 6.02×10²³ molecule of sucrose

Hence, one mole of different compounds has different masses but has the same number of molecules.

When we take into consideration the ions, then

96 g of SO₄^2-   = 1 mol of SO₄^2-  = 6.02×10²³ ions of SO₄²

 62 g of NO₃^–  = 1 mol of NO₃^–  = 6.02×10²³ ions of NO₃^–  

From the above discussion, we reach the conclusion that the number 6.02×10²³ refer to one mole of a substance. This number is called Avogadro’s number and it is denoted by N_A .

Following relationships between amounts of substances in terms of their masses and the number of particles present in them, are usefull.

Number of atoms of an element = Mass of the element x N_A / Atomic mass 

Number of molecules of a compound = Mass of compound x N_A  / Molecular mass

Number of ions of ionic species  = Mass of the ion x N_A / Ionic mass

When we have compounds of known mass we can calculate the number of atoms from their formulae.

In 18 g of water, there are 6.02×10²³ molecules of water, 2 x 6.02×10²³ atoms of hydrogen, and 6.02×10²³ atoms of oxygen. Similarly, 98 g H₂SO₄ has twice Avogadro’s number of hydrogen atoms, four times Avogadro’s number of oxygen atoms, and Avogadro’s number of Sulphur atoms.

Some substances ionize in stable solvents to yield cations and anions. The number of such ions, their masses, number of positive and negative charges can be easily calculated from the known amount of the substance dissolved.

Let us dissolve 9.8 g of H₂SO₄ in a sufficient amount of water to get it completely ionized. It has 0.1 moles of H₂SO_4. It will yield 0.2 mole or 0.2 x 6.02×10²³ H⁺ and 0.1 moles or 0.1 x 6.02×10²³ SO₄^2- etc.

The total positive charges will be 0.2 x 6.02×10²³ and the total negative charges will be 0.2 x 6.02×10²³ (because each SO₄^2- , has two negative charges).

The total mass of H⁺ is (0.2 x 1.008) and that of SO₄^2- is (0.1 x 96) g.

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