Molecular Mass And Mole Concept

Key notes :

Molecular Mass:

• Definition: Molecular mass is the mass of one molecule of a substance, calculated by adding the atomic masses of the atoms in the molecule.
• Unit: It is expressed in atomic mass units (amu) or g/mol.
• Example: The molecular mass of water (H₂O) is:

Molecular mass of H₂O = (2×1) + (1×16) = 18 amu

• Importance: It helps in calculating the amount of substance needed for reactions and determining the number of molecules.

Mole Concept:

• Definition: A mole is a quantity used to count particles (atoms, molecules, ions, etc.). One mole of any substance contains 6.022 × 10²³ particles (Avogadro’s number).
• Unit: Moles are expressed in mol.
• Formula:

Number of moles = Mass of substance (in grams) / Molar mass (g/mol)​

• Example: If 18 grams of water (H₂O) is taken, the number of moles of water is:

Moles of H₂O = 18 g / 18 g / mol = 1 mole

Molar Mass:

• Definition: The molar mass is the mass of one mole of a substance, usually in units of grams per mole (g/mol).
• Relationship with Molecular Mass: Molar mass (g/mol) is numerically equal to molecular mass (amu), but the unit is grams per mole instead of atomic mass units.
• Example: For water (H₂O), the molecular mass is 18 amu, and the molar mass is 18 g/mol.

Avogadro’s Number:

• Definition: Avogadro’s number (6.022 × 10²³) is the number of particles in one mole of a substance.
• Example: One mole of carbon atoms contains 6.022 × 10²³ carbon atoms.

Mole and Chemical Reactions:

• Stoichiometry: The mole concept is essential in stoichiometry to calculate the amount of reactants and products in a chemical reaction.
• Balanced Equation: The coefficients in a balanced chemical equation represent the number of moles of each reactant and product involved.

Conversions:

• From grams to moles: Use the formula:

Moles = Mass (g) / Molar mass (g/mol)

• From moles to molecules: Use Avogadro’s number:

Number of molecules = Moles × (6.022 × 10²³)

Application in Chemistry:

• Empirical Formula: The mole concept helps determine the simplest whole-number ratio of elements in a compound.
• Molecular Formula: The actual number of atoms of each element in a molecule, determined using moles.

Summary:

• The mole concept links the atomic and macroscopic worlds.
• Molecular mass is the mass of a molecule, and molar mass is the mass of one mole of molecules.
• The mole is a convenient way to express quantities of substances involved in chemical reactions.