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Mean molecular weight (μ) is a useful measure of the molecules of a gas, which can be used in some useful approximations that allow tractable models of the gas, much as the Rosseland mean opacity does for opacity. It is useful in devising tractable radiative transfer models or equations of state, helpful for numerical models and perhaps even more helpful for analytic models. (Despite the term weight, it is a measure of their mass.) Useful units are atomic mass units (amu) or grams per mole.