1. Mole Concept

The mole is a fundamental unit in chemistry used to express amounts of a chemical substance. It is defined as the amount of substance that contains as many elementary entities (atoms, molecules, ions, etc.) as there are atoms in 12 grams of pure carbon-12 (12C).



Avogadro's Number (Nₐ): This is the number of atoms in exactly 12 grams of carbon-12, which is 6.022 × 10^ {23}. This number is known as Avogadro's constant.

Mole (mol): One mole of any substance contains 6.022 ×  10^{23} entities (atoms, molecules, formula units, etc.).

Key Points:

1. 1 mole of atoms of an element = 6.022 ×  10^{23} atoms.

2. 1 mole of molecules of a compound = 6.022 × 10^ {23} molecules.

3. 1 mole of ions of an ion = 6.022 × 10^ {23} ions.

2. Molar Mass

Molar mass is the mass of one mole of a substance (element or compound). The molar mass of an element is the mass of one mole of atoms of that element, and it is numerically equal to the atomic mass of the element expressed in grams.

Calculation:

Molar Mass of an Element: The atomic mass of the element in grams per mole (g/mol).

Molar Mass of a Compound: The sum of the molar masses of the individual elements in the compound, multiplied by the number of times each element is present in the compound.

For example, the molar mass of water (H₂O) can be calculated as:

Molar Mass of H₂O = 2 × {Atomic Mass of H} + (1 × {Atomic Mass of O}

= (2 × 1.008 g/mol) + (1 ×  16.00 g/mol)

= 2.016 /mol + 16.00 g/mol

= 18.016 g/mol

Also Read: Electrochemistry Notes for Class 12 JEE MAIN and Advanced

3. Applications of the Mole Concept

Stoichiometry: Using the mole concept, we can calculate the amounts of reactants and products involved in a chemical reaction.

Determining Empirical and Molecular Formulas: The empirical formula of a compound gives the simplest whole-number ratio of the elements in the compound, while the molecular formula gives the actual number of atoms of each element in a molecule.

Gas Laws: The mole concept is essential in understanding and applying the ideal gas law, where one mole of an ideal gas occupies 22.4 liters at standard temperature and pressure (STP).

4. Practice Problems

1. Calculating Moles from Mass:

Problem: How many moles are in 25 grams of water (H₂O)?

Solution: Molar mass of H₂O = 18.016 g/mol

Number of moles = Mass/Molar Mass

= 25g/18.016g/mol

= 1.387 mol

2. Calculating Mass from Moles:

Problem: What is the mass of 3 moles of carbon dioxide (CO₂)?

Solution: Molar mass of CO₂ = 44.01 g/mol

Mass = Number of moles × Molar Mass

= 3 mol×  44.01/mol

= 132.03g

3. Calculating Number of Entities from Moles:

  Problem: How many molecules are there in 2 moles of oxygen gas (O₂)?

  Solution:

Number of molecules = Number of moles × N_A

 = 2mol × 6.022× 10^{23} molecules/mol

 = 1.204 ×  10^{24} molecules}

These notes cover the fundamental concepts and calculations involving the mole and molar masses, crucial for understanding and performing chemical reactions and stoichiometric calculations in chemistry.