Atoms and Molecules

Mole of Molecules

1 mole of molecules of a substance has a mass equal to the gram molecular mass of the substance.That is,
1 mole of molecules of a substance = Gram molecular mass of the substance
Now, the molecular mass of oxygen ($O$₂) is 32 u, so the gram molecular mass of oxygen is 32 grams.Thus,
1 mole of oxygen molecules = Gram molecular mass of oxygen
= 32 grams
It should be noted that the molecular formula of a substance represents 1 mole of molecules of that substance. For example :
(i) Formula $O$₂ represents 1 mole of oxygen molecules and $O$_2O represents 2 moles of oxygen molecules
(ii) Formula $H$₂O represents 1 mole of water molecules and $2H$₂O represents 2 moles of water molecules
Please note that one mole of sulphuric acid, $H$₂SO₄, contains :
2 moles of hydrogen atoms
1 mole of sulphur atoms
and 4 moles of oxygen atoms
Knowing the mass of 1 mole of a substance we can calculate the mass of any number of moles of that substance.The masses of the moles of some of the substances are given below :

Substance	Symbol or Formula	Mass of 1 mole (Molar mass)	This mass (1 mole) contains
1 mole of hydrogen atoms 1 mole of hydrogen molecules 1 mole of water molecules 1 mole of ammonia molecules 1 mole of sodium atoms	H $H$2 $H$2O $NH$3 Na	1 g 2x1=2 g 2 + 16 = 18 g 14 + 3 = 17 g 23 g	$6.022X1023$ hydrogen atoms $6.022X1023$ hydrogen molecules $6.022X1023$ water molecules $6.022X1023$ ammonia molecules $6.022X1023$ sodium atoms

We are now in a position to give the definition of mole as a unit for the amount of substance.A mole of a substance is that amount of the substance which contains the same number of particles (atoms, molecules or ions) as there are carbon atoms in 12 grams of carbon-12 element.Since 12 grams of carbon-12 element contain $6.022X1023$ atoms of carbon, we can also say that: A mole of a substance is that amount of the substance which contains $6.022X1023$. particles (atoms, molecules or ions) of the substance. Thus, the SI unit of amount of a substance is mole which is written in short form as mol. From the above discussion we conclude that:
A mole represents two things :
1.A mole represents a definite amount of the substance.It represents the amount of a substance equal to its gram atomic mass or gram molecular mass.
2.A mole represents a definite number of atoms, molecules or ions of a substance. It represents $6.022X1023$ atoms, molecules or ions of a substance.
We will discuss some problems based on the moles of atoms and moles of molecules.