MW = Σ (number of atoms × atomic weight)
Molecular weight (MW) is numerically identical to molar mass — it's the sum of atomic weights of all atoms in a molecule, expressed in daltons (Da) or g/mol. For ionic compounds, the term "formula weight" is more accurate since no discrete molecules exist. MW is directly measured by mass spectrometry: the molecular ion peak [M]⁺ or [M+H]⁺ gives the exact mass. High-resolution MS can determine MW to 4+ decimal places, enabling molecular formula determination. For proteins, MW ranges from ~5,000 Da (insulin) to millions (titin, ~3.8 MDa). Techniques like SDS-PAGE, gel filtration, and MALDI-TOF MS are used for MW determination of biological macromolecules. Average MW accounts for isotope distributions.