Tag Archives: Limiting reagent

Mole Concept

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.


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 NA i.e. NA = 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 Naion    = 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 Hreact with 1 mole of Oto produce 2 moles of water.

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

III. 44.8 L of Hreact with 22.4 L of Oto produce 22.4 L of water vapours.

IV. 4 g of Hreact with 32 g of Oto 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.

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