Concept of the Mole

The mole is the SI unit for amount of substance, defined such that one mole contains exactly 6.02214076×10²³ elementary entities (atoms, molecules, ions, electrons, etc.). It provides a bridge between the atomic scale and macroscopic measurements.

Avogadro’s Number

Avogadro’s number (N_A = 6.02214076×10²³) is the fixed numerical value of entities per mole. It allows conversion between number of particles and moles:

Number of moles = (Number of entities) ÷ N_A

Mole–Mass Relationship

The molar mass (g/mol) of a substance equals its relative atomic or molecular mass. Conversions:

• Moles = mass (g) ÷ molar mass (g/mol)
• Mass = moles × molar mass

Mole and Gas Volume at STP

At standard temperature and pressure (0 °C, 1 atm), one mole of an ideal gas occupies 22.414 L. Thus:

• V (L) = moles × 22.414
• Moles = V (L) ÷ 22.414

Molarity & Solution Calculations

Molarity (M) is moles of solute per liter of solution:

M = moles of solute ÷ volume of solution (L)

To prepare solutions, calculate required mass:

Mass (g) = M × volume (L) × molar mass (g/mol)

Numerical Calculation Examples

1. Calculate moles in 5.0 g of NaCl (M = 58.44 g/mol).
2. Determine mass of 0.250 mol H₂O (M = 18.02 g/mol).
3. Find volume at STP of 2.00 mol CO₂.
4. Prepare 500 mL of 1.00 M glucose solution (C₆H₁₂O₆, M = 180.16 g/mol).

Common Pitfalls & Tips

• Ensure units match (grams vs. liters).
• Watch for significant figures in constants.
• Convert mass to moles before volume in gas calculations.
• Use proper molar mass (sum of atomic masses from periodic table).

Conclusion

Mastering the mole concept and related calculations is fundamental to quantitative chemistry. With practice, converting between mass, moles, and volume becomes intuitive, enabling precise experimental design and analysis.