What is the unit of entropy?

Fill in the blanks:
$(i)$ The entropy of one mole of a substance at a fixed temperature and standard state is called ........ .
$(ii)$ The unit of molar entropy is .......... .
$(iii)$ The process of conversion of graphite into diamond is of ........ type.

State the limitation of the second law of thermodynamics.

What is the entropy change (in $J\, K^{-1}\, mol^{-1}$) when one mole of ice is converted into water at $0^{\circ}C$? (The enthalpy change for the conversion of ice to liquid water is $6.0\, kJ\, mol^{-1}$ at $0^{\circ}C$)

Which one of the following involves a decrease in entropy?

For a $Carnot$ engine,the source is at $500 \ K$ and the sink at $300 \ K$. What is the efficiency of this engine?