Alright, so now I understand that entropy is a state function and we can easily calculate it by devising a reversible path between the two states of an irreversible path. For calculating the entropy of mixing of two different gases, the professor in this video at 35:02 states that we can achieve such a reversible process by using pistons permeable only to each individual gas, but, how is this physically possible? Does there really exist such pistons permeable only to one kind of gas or is this whole procedure just a mathematical construct?
When I say mathematical construct, I mean that it is just a mathematical piston that we could use to drive the process backward and calculate entropy i.e: does not exist in reality.
I discussed this problem with a friend of mine, and he told to me that we could achieve this if we had a hypothetical porous wall that had intermediate pore size between the molecule size of two gases. So, the gas with a lower pore size could simply pass through the porous wall.
So, is this experimentally feasible? If they do exist, then I'd appreciate some videos demonstrating it.