For a sample of perfect gas when its pressure is
changed isothermally from ``p_i`` to ``p_f``, the
entropy change is given by
(1)``\triangle S`` = nRT `` ln \frac {p_f}{p_i}``
(2)``\triangle S`` = RT `` ln \frac {p_i}{p_f}``
(3)``\triangle S`` = nR `` ln \frac {p_f}{p_i}``
(4)``\triangle S`` = nR `` ln \frac {p_i}{p_f}``