Molar mass is the mass of one mole of a substance (atoms, molecules, or ions). It tells us how many grams one mole of that substance weighs.
- The unit of molar mass is grams per mole (g/mol).
- Numerically, the molar mass of an element or compound is equal to its relative atomic mass (Ar) or molecular mass (Mr), but expressed in grams.
Examples:
- Hydrogen (H): Atomic mass ≈ 1 → Molar mass = 1 g/mol
- Oxygen (O): Atomic mass ≈ 16 → Molar mass = 16 g/mol
- Water (H₂O): (2 × 1) + 16 = 18 → Molar mass = 18 g/mol
- Sodium chloride (NaCl): 23 + 35.5 = 58.5 → Molar mass = 58.5 g/mol
This means:
- 1 mole of H atoms = 1 g
- 1 mole of O atoms = 16 g
- 1 mole of H₂O molecules = 18 g
- 1 mole of NaCl formula units = 58.5 g
In short, molar mass connects the microscopic world (number of particles) with the macroscopic world (mass in grams measured in the lab).