Adiabatic process is isentropic, but iI can't get it why is it so. If we go by statistical method S=KlnW(where W
$$S = K\ln W,$$
where $W$ is the thermodynamic probability), soand if we look into adiabatic expansion the volume increases so, we can say that the thermodynamic probability increases, and hence the entropy of the system should increase.
Moreover, if we look into isothermal expansion of an ideal gas in vaccumvacuum, there would be no work done, and so no heat would enter or move out of the system but. But since there is a change in volume, we can write ∆S=nRln(V2/V1).
$$∆S = nR\ln\frac{V_2}{V_1}.$$