Water and alcohol forms an azeotrope where the blend will carry over with the vapor having the same concentration as solution at all times. The azeotrope is 96%/4% by weight.
From reading through wikipedia, if the starting concentration has more EtOH than azeotrope, the final product will be slightly higher in ethanol than azeotrope and slightly below if its below azeotrope to start.
Suppose you are distilling 50/50 blend of water ethanol, but the final product sought after is the water. What is the maximum purity of water achievable here and how do we know this?