The molecular weight of an element, molecule or ion divided by its valence (or valence change for a redox reaction). For example, the molecular weight of calcium hydroxide, or "slaked lime," (Ca(OH)₂) is 72. 10. Because the valency of calcium in this case is 2, the equivalent weight of lime is 36. 05. Mud analyses give concentrations in various units: ppm, mg/L, wt. % and epm. Mud engineers should recognize the meaning of epm and equivalent weight of a mud chemical.