For a sample of perfect gas when its pressure is changed isothermally from `p_(i)` to `p_(f)` , the entropy change is given by

Options

A. `DeltaS=nR "In" ((P_(f))/(P_(i)))`
B. `DeltaS=nR "In" ((P_(i))/(P_(f)))`
C. `DeltaS=nRT"In" ((P_(f))/(P_(i)))`
D. `DeltaS=RT "In" ((P_(i))/(P_(f)))`

Answer

B

Explanation

Entropy change is given as,
`DeltaS=nC_(p) "In "(T_(f))/(T_(i))+nR "In" (P_(i))/(P_(f))" "…(i)`
For isothermal process, `T_(i)=T_(f)`
`therefore " "nC_(p)"In" (T_(f))/(T_(i))=nC_(p)"In" (T_(i))/(T_(f))=0" "["In" 1=0]`
For Eq. (i) `DeltaS=nR "In" (P_(i))/(P_(f))`