<p>For a sample of perfect gas when its pressure is changed isothermally from&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>p</mi><mi>i</mi></msub><mo>&#160;</mo><mi>t</mi><mi>o</mi><mo>&#160;</mo><msub><mi>p</mi><mi>f</mi></msub></math>, the entropy change is given by:</p>