Mole Concept and Molar Masses
Learning Objective: Understand the meaning of a mole, how to relate it to mass, volume, number of particles, and how to use molar mass in calculations.
Quick Notes:
- A mole is the amount of substance that contains 6.022 × 1023 particles (Avogadro’s number).
- These particles may be atoms, molecules, ions, or electrons.
- Molar mass = mass of 1 mole of a substance (same as molecular/formula mass in grams).
- Key relationships:
- 1 mole = molar mass (g)
- 1 mole = 6.022 × 1023 entities
- For gases at STP: 1 mole = 22.4 L
- Use mole concept to calculate:
- Mass from moles
- Number of particles from moles
- Volume of gas at STP
Full Notes:
What is a Mole?
In chemistry, substances are made up of atoms, molecules, or ions. Since these particles are extremely small, we use the concept of the mole to count them in practical amounts.
Definition:
A mole is the amount of a substance that contains as many entities as there are atoms in exactly 12 grams of carbon-12.
That number is known as Avogadro’s number:
NA = 6.022 × 1023 entities/mol
These entities can be atoms, molecules, ions, electrons, etc.
This standardised “counting unit” allows chemists to compare and measure substances reliably in chemical reactions.
Molar Mass
The molar mass of a substance is the mass of one mole of that substance. It is expressed in grams per mole (g/mol).
Numerically, molar mass is equal to:
- The atomic mass (for atoms)
- The molecular mass (for molecules)
- The formula mass (for ionic compounds)
But instead of being in amu, it's expressed in grams.
Never confuse molar mass with molecular mass in amu, or the mass of 1 mole with the mass of a single particle.
Relationship Between Moles, Mass, and Number of Entities
We can use the mole concept to interconvert between:
- Mass ↔ Moles
- Number of particles ↔ Moles
- Gas volume at STP ↔ Moles
Key formulas:
Moles = Mass (g) / Molar mass (g/mol)
Entities = Moles × Avogadro’s number
Worked Examples
Example 1: Calculate the number of moles in 75.0 g of water.
- Molar mass of H2O = 18.02 g/mol
- Moles = 75.0 ÷ 18.02 = 4.16 mol
Example 2: How many molecules are present in 1.00 g of hydrogen gas (H2)?
- Molar mass of H2 = 2.016 g/mol
- Moles = 1.00 ÷ 2.016 = 0.496 mol
- Molecules = 0.496 × 6.022 × 1023 ≈ 2.99 × 1023 molecules
Application in Stoichiometry
In chemical equations, coefficients represent moles of substances.
Example:2H2 + O2 → 2H2O
This means:
- 2 moles of hydrogen gas react with 1 mole of oxygen gas to produce 2 moles of water.
By using molar masses and Avogadro’s number, we can:
- Calculate the mass of reactants and products
- Determine the volume of gases involved
- Find the number of atoms or molecules
Summary
- 1 mole contains 6.022 × 1023 entities (Avogadro’s number).
- Molar mass in g/mol equals the relative atomic or molecular mass in amu.
- Relationships connect moles with mass, gas volume, and particle number.
- Mole concept is the foundation for stoichiometric calculations.