Osmotic pressure, question on derivation (TD)

I'm currently reading about thermodynamics and osmosis and I happened to stumble across this paper. There is one thing I don't really understand, though.. In chapter 8 the author wishes to give a thermodynamic explanation of the osmotic pressure so I've been reading through the derivation. When calculating the thermodynamic probability W (page 20), how do I go from (11) to (12)?

http://arxiv.org/ftp/physics/papers/0305/0305011.pdf