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

Options

A. `DeltaS=nRTln((p_(f))/(p_(i)))`
B. `DeltaS=nRln((p_(i))/(p_(f)))`
C. `DeltaS=nRTln((p_(f))/(p_(i)))`
D. `DeltaS=RTln((p_(i))/(p_(f)))`.

Answer

B

Explanation

For the expansion of n mol of an ideal gas, the change in entropy of reversible isothermal process is
`DeltaS=nR"ln"(V_(f))/(V_(i))`
At constant temperature, for n mol of ideal gas, `(V_(f))/(V_(i))=(p_(i))/(p_(f))`
Therefore, `DeltaS=nRln((P_(i))/(p_(f)))`.