Mole : One mole is the amount of a substance which contains 6.022 x 10²³ constituent particles of the substance. These particles may be atoms, molecules or ions.

Or

One mole can also be defined as the amount of a substance which contains as many particles as the number of C-12 atoms in its 12 g mass.

The number 6.022 x 10²³  is called Avogadro number or constant and it is denoted by N or N_A i.e. N_A = 6.022 x 10²³

Atomic Mass Unit(amu) : In Chemistry, we have to make calculations with extremely small and big numbers especially the mass of an atom.

Molar Mass : The mass of one mole of a substance is called its molar mass.

Ex: Molar mass of CaCO₃       = 100g

Molar mass of Na atom  =  23 g

Molar mass of Na⁺ ion    = 23 g

Percentage Composition: As we know that each element of a compound holds a fixed percentage by mass.

% of an element = Mass of an element × 100/Molecular or Formula mass of the compound

Empirical formula : The empirical formula of a compound represents simplest whole number ratio of atoms present in it. Ex: The empirical formula of Benzene(C₆H₆) is CH.

Molecular formula : The molecular formula of a compound represents the actual number of atoms present in the compound. Ex: The molecular formula for Benzene is C₆H₆.

Key Point : Molar mass = Empirical formula mass x n

Stoichiometry : The study of calculations on the basis of chemical reactions is called stoichiometry.

We can get a lot of  information from an equation of a chemical reaction. Consider a chemical equation or reaction :

2H₂ (g)+  O₂ (g)→2H₂O (g)

We can make following observations:

I. 2 moles of H₂ react with 1 mole of O₂ to produce 2 moles of water.

II. 2 molecules of H₂ reacts with 1 molecule of O₂ to produce 2 molecules of water.

III. 44.8 L of H₂ react with 22.4 L of O₂ to produce 22.4 L of water vapours.

IV. 4 g of H₂ react with 32 g of O₂ to produce 36 g of water vapours.

Limiting Reagent : In a chemical reaction, the reactant which is present in the lesser amount is called the limiting reagent. In actual, a limiting reagent controls the reaction as no reaction takes place after it is consumed.

Ex : Let us take 2 g hydrogen and 20 g oxygen to form water.

According to the reaction  2H₂ +  O₂ →2H₂O

We know that 4 g of hydrogen requires 32 g of oxygen to form water which means that 2 g of hydrogen will react with 16 g of oxygen. So, 4 g oxygen will be left unreacted. It’s obvious that hydrogen is the limiting reagent.