1. The problem statement, all variables and given/known data The question is says: Two vessels divided by a partition contain 1 mol of N2 and 2 mol of O2 gas. If the partition is removed and gases ate mixed isothermally, then find the change in entropy due to mixing assuming initial and final pressure are same . 2. Relevant equations ΔS=qrev/T [the one I know] ΔSsys=-RΣniln xi Where ni = number of moles of gas. xi= mole fraction of gas.[ the foreigner equation] 3. The attempt at a solution As written here in the relevant equations part, the second equation is the one which is new to me. I don't know how it works, what it does and from where it came from. It looks like boltzmann's entropy formula but I don't think that both of them are same. The question can be easily solved by this equation, but this equation is the one creating problem.