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Molar mass (M) is the mass of one mole of a substance in grams per mole. It equals the sum of atomic masses of all atoms in the formula: M = Σ(nᵢ × Aᵢ), where nᵢ is the number of atoms and Aᵢ is the atomic mass from the periodic table. Examples: H₂O = 2(1.008) + 16.00 = 18.015 g/mol; NaCl = 22.99 + 35.45 = 58.44 g/mol; C₆H₁₂O₆ = 6(12.011) + 12(1.008) + 6(16.00) = 180.16 g/mol. Molar mass is fundamental for converting between mass and moles in stoichiometric calculations, preparing solutions of known molarity, and interpreting mass spectrometry data. For polymers and proteins, the average molar mass may be expressed as number-average (Mn) or weight-average (Mw).