Atom-based approaches for predicting this quantity are named log [this quantity] prefixed by capital A, X, or M. Initial composition of an alloy multiplied by quantity mass raised to the power of this quantity (-5[1])minus 1 gives the final composition during solidification by the Scheil (“shyle”) equation. For one class of substances, there is a nearly linear relationship (15[1])between this quantity and EC50 by the Meyer-Overton rule. This quantity is inversely proportional to the MAC (“M-A-C”) for (*) general anesthetics. This quantity only takes into account unionized species, unlike a related quantity symbolized capital D. The lipophilicity of a drug is often judged by its value for this quantity for a two-phase system consisting of n-octanol and water. For 10 points, name this quantity, the ratio of equilibrium concentrations of a given compound in two immiscible solvents. ■END■