Molecules calculation

Mass of a molecule is sum of the masses of the constituent atoms.
The mass of an atom is sum of masses of neutrons and protons.
In turn, masses of certain number of molecules too would depend on the masses of the constituent atoms.
Using this logic, Avogadro devised a special term 'mole' which is a certain mass equivalent to the molar mass expressed in grams.

Introduction :

Avogadro was the first person to establish that elements exist as molecules.
Thus a mole can be defined as a certain number of molecules. This number was found out to be 6.022 x 10²³ molecules.
Thus a mole of oxygen, hydrogen, carbon dioxide, carbon monoxide or any substance would contain 6.022 x 10²³ molecules.
It can be further illustrated as under
If two moles of hydrogen combines with one mole of oxygen what is the number of molecules of water formed?
One mole= 6.022 x 10²³ molecules.
Two moles= 12.044x 10²³ molecules
                  2H₂            +                                O₂ ---------->                 2H₂O
12.044x 10²³ molecules + 6.022 x 10²³ molecules.---->          2 moles
So the answer is 12.044x 10²³ molecules of water. It may be mentioned here that it is not 2+1=3, as in mathematics.

Illustrations of Molecules Calculation

Let us consider a few problems.
Question 1;- How many molecules are there in 56 g of carbon monoxide?
Ans: First let us find the number of moles

number of moles = mass in gram/molar mass
=56/28
=2 moles
Now by Avogadro's hypothesis,
One mole of a gas contains 6.022 x 10²³ molecules.
so two moles would contain
2 x 6.022 x 10²³ molecules.
12.044x 10²³ molecules.

Question 2 :-
Find the moles in12.044x 10²³ molecules of hydrogen at STP.
Ans:  One mole of hydrogen = 6.022 x 10²³ molecules.
so 12.044x 10^23 /6.022 x 10²³ 
=2 moles
Question 3
A gas has volume of 44.8 L. What is the number of molecules in it?
22.4 L volume is occupied by one mole i.e. 6.022 x 10²³ molecules.
So, 44.8 L would be occupied by   6.022 x 10²³ x 2
= 12.0444 x 10²³ molecules.