In chemistry, the standard molar entropy is the entropy content of one mole of substance, under standard conditions (not standard temperature and pressure STP).
The standard molar entropy is usually given the symbol S°, and has units of joules per mole kelvin (J mol−1 K−1). Unlike standard enthalpies of formation, the value of S° is an absolute. That is, an element in its standard state has a nonzero value of S° at room temperature. The entropy of a pure crystalline structure can be 0 J mol−1 K−1 only at 0 K, according to the third law of thermodynamics. However, this presupposes that the material forms a 'perfect crystal' without any frozen in entropy (defects, dislocations), which is never completely true because crystals always grow at a finite temperature. This residual entropy is often quite negligible.
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