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

For a sample of perfect gas when its pressure is changed isothermally

1.	For a sample of perfect gas when its pressure is changed isothermally from p_(i) to p_(f) , the entropy change is given by
Answer» `DeltaS= nRln((p_(F))/( p_(i)))` `DeltaS= nRln((p_(i))/( p_(f)))` `DeltaS= nRTln((p_(f))/( p_(i)))` `DeltaS= nRTln((p_(i))/( p_(f)))` Solution :For n moles of an ideal gas UNDERGOING reversible EXPANSION when temperature changes from`T_(1)` to`T_(2)`and pressure changes from `p_(1)` to `p_(2)`, ENTROPY change is given by `DeltaS = n C_(p) ln. (T_(2))/(T_(1)) + nRln. (P_(1))/(P_(2)) ` ( Refer to page)or `DeltaS = nC_(p)ln. (T_(f))/(T_(i))+ nRTln. (p_(i))/(p_(f))` For isothermal expansion, `T_(i) = T_(f)` so that `ln. (T_(f))/(T_(i)) = ln 1= 0`. Hence, `DeltaS = nR ln. (p_(i))/( p_(f))`