During a fractional distillation of a 25mL ethanol/H2O soln, it was found that first the temperature stayed constant at ~75 C for 3mL of soln, then it went up to 100 C for the rest of the soln.

The first question is:
Assuming that the unknown (ethanol) did not form an azeotrope, the beginning percent composition of ethanol/H2O soln would be:

I put: (3/25)*100 = 12% composition

The second question is:
If the unknown (ethanol) DID form an azeotrope w/ water, the beginning percent composition of ethanol/H2O soln would be:

I have no idea how to answer this question.

To answer the second question, we need to understand what an azeotrope is. An azeotrope is a mixture of liquids that boils at a constant temperature and with a constant composition. In the case of ethanol and water, there is an azeotrope known as the ethanol-water azeotrope.

If ethanol did form an azeotrope with water, it means that at a certain composition, the mixture would boil at a constant temperature. In this case, the temperature remained constant at approximately 75°C for the initial 3mL of the solution. This suggests that this portion of the solution contained the azeotrope.

To determine the percent composition of ethanol in this azeotropic mixture, we can use the information about the boiling points of pure ethanol and pure water. The boiling point of pure ethanol is approximately 78.3°C, while the boiling point of pure water is 100°C. Since the temperature stayed constant at around 75°C, it implies that the mixture boiling at this temperature contains a lower percentage of ethanol compared to pure ethanol.

To estimate the percent composition of ethanol, we can use the temperature difference between the boiling points of pure ethanol and pure water. The total temperature range is 100°C - 75°C = 25°C, and the boiling point difference between ethanol and water is approximately 21.7°C. We can assume that the percentage difference in composition is proportional to the temperature difference.

So, to estimate the percent composition of ethanol in this case, we calculate:

Percent composition = (Temperature difference / Total temperature range) * 100

Percent composition = (21.7°C / 25°C) * 100

Percent composition = 86.8%

Therefore, if the unknown ethanol formed an azeotrope with water, the beginning percent composition of the ethanol/water solution would be approximately 86.8%.