Question

1. Why is it impossible for greater than 96% pure ethanol to be obtained from fractional distillation of the fermentation mixture?

2. In terms of the velocity of light, what is refractive index?

3. Does your graph of the simple and fractional distillations show any difference in the shapes of the two curves? What does this difference, if any; indicate about the efficiency of fractional distillation relative to that of simple distillation for separating the ethanol/water azeotrope from water?

4. A compound has a refractive index of 1.3191 at 20.0oC. Calculate its refractive index that would be expected at 25.0oC.

Answer 1

1. For fermentation mixture, it is impossible to do fractional distillation greater than 96 % ethanol, because water and ethanol forms an azeotrope at concentration of 95 % ethanol. This azeotrope has boiling point lesser than that of ethanol, thus it boils before ethanol. This means that we get azeotrope mixture on the other side of the distillation, which means the ethanol does not remain pure.

2. Refractive index is of a material is ratio of velocity of light in vacuum to that in dispersive medium.

refractive index = n = c/v

c = speed of light in vacuum

v = velocity in medium

3. The shapes of simple and fractional distillations are different. Both are sigmoidal curves, but fractional distillation has larger spike. The graph tells that the fractional distillation has much larger jump than simple distillation. Initially mostly alcohol will evaporate leaving water behind but after that we have just water. In simple distillation, the process is very gradual. This tells that frcational is more efficient than simple distillation.

4. The refractive index is inversely proportional to temperature.

refractive index = n_o = 1.3191, n_T = ?

T_o = 20.0 ^oC = 293 K, T = 25.0 ^oC = 298 K

=1.3137

refractive index at 25 ^oC = 1.3137