Molar entropy denotes the entropy content of one mole of any substance. When entropy content is measured for each mole at steady state (298 K temperature), it is referred to as standard molar entropy.
Entropy is the measure of degree of disorder of a system. Molar entropy of a substance is therefore degree of disorder or chaos in one mole of that substance. For any system in equilibrium, the entropy is zero. This is because entropy measures the degree of activity or motion of molecules in the system. The concept of entropy bears resemblance to that of energy, in that both serve to drive physical or chemical changes in any substance. When a substance undergoes change, not involving either exchange or transfer of energy, it is due to a change in the entropy of that particular system. Molar entropy is thereby studied in the field of thermodynamics. This is to account for changes or activities in any substance that is unaccounted for by energy changes alone.
Molar entropies have been measured under steady conditions for many substances. These values have allowed few common trends to be established regarding them. Molar entropy is highest for compounds in gaseous state, and is lowest for solids. As atomic mass increases, molar entropy value increases. Therefore, molar entropy is higher for heavier and more complex molecules. Generally, any reaction or natural process results in an increase in entropy. Similarly, an increase in entropy can result in an unpredictable reaction or natural process that is not explained by a change in energy.