Weight-average Molar Mass

Weight-average molar mass is a polymer characterization measure that describes the average molar mass of molecules while giving greater weight to larger chains, making it sensitive to molecular-weight distribution. It is calculated by weighting each molecular species according to its mass contribution, commonly expressed as Mw = ΣNiMi²/ΣNiMi, where Ni is the number of molecules and Mi is the molar mass of each species. In chemistry, comparing weight-average molar mass with number-average molar mass helps determine dispersity and assess how broadly chain lengths vary. Measurements such as size-exclusion chromatography and light scattering use these values to evaluate polymer synthesis, processing behavior, and material performance.

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JoVE Core - Chemistry

Molar Mass

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2020

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The volume occupied by one mole of a substance is its molar volume. The ideal gas law, PV = nRT, suggests that the volume of a given quantity of gas and the number of moles in a given volume of gas vary with changes in pressure and temperature. At standard temperature and pressure, or STP (273.15 K and 1 atm), one mole of an ideal gas (regardless of its identity) has a volume of about 22.4 L — this is referred to as the standard molar volume. For example, one mole each of hydrogen, oxygen,...

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Polymer samples typically consist of macromolecular chains with a distribution of lengths, resulting in a range of molar masses rather than a single discrete value. Conventional descriptors such as the number-average molar mass and weight-average molar mass quantify this distribution but do not fully capture polymer behavior in solution..The viscosity-average molar mass provides a more realistic description of polymer behavior in solution because it accounts for the enhanced contribution of...

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