You collected the first drop from the glycerol – water distillation in a collection flask. Look at the boiling point composition curve. What mole percent water should be in the first drop collected? a ) 1 0 0 mole percent water b ) 9 0 mole percent water d ) 7 0 mole percent water
Answer
Glycerol-Water Distillation: Mole Percent in First Drop
In the distillation of a binary mixture like glycerol and water, the component with the lower boiling point distills first. Water has a boiling point of approximately 100°C, while glycerol has a much higher boiling point of around 290°C.
🌡️ Interpreting the Boiling Point–Composition Curve
A boiling point–composition curve for glycerol and water shows how the composition of the vapor changes during distillation. At the start of the distillation:
- Water vaporizes first because it has a much lower boiling point than glycerol.
- The vapor is rich in the more volatile component, i.e., water.
🔬 Composition of the First Drop
The first drop of distillate collected is essentially the condensed vapor of the most volatile component. Since water is more volatile than glycerol, the vapor in the early stages of the distillation is almost entirely composed of water.
a) 100 mole percent water
🧪 Summary
The reason the first drop is nearly pure water is because during simple distillation, the component with the lowest boiling point dominates the vapor phase at the beginning. Only as distillation progresses does the composition start to shift.