Molar mass links the atomic/molecular scale to the macroscopic world. It equals the sum of atomic masses of all atoms in a molecule.
Examples
- H₂O: (2×1.008) + 16.00 = 18.015 g/mol
- CO₂: 12.01 + (2×16.00) = 44.01 g/mol
- NaCl: 22.99 + 35.45 = 58.44 g/mol
Moles Formula
n = mass / molar mass. If you have 36.03 g of water: n = 36.03/18.015 = 2 moles.
Molar Mass and the Periodic Table
Every element's atomic weight, listed on the periodic table, is essentially its molar mass in g/mol — carbon's atomic weight of 12.011 means one mole of carbon atoms weighs 12.011 grams. This direct readability is what makes the periodic table the starting point for virtually all stoichiometry calculations.
Common Calculation Pitfall
Forgetting to multiply by the subscript count in a formula is the most frequent student error — for a compound like Ca(OH)₂, you need 1 calcium + 2 oxygen + 2 hydrogen atoms' worth of mass, not just one of each, since the subscript outside the parentheses applies to everything inside it.