Chapter 5

Question 1

Calculate how much carvone will distill for every 1.0g of water that codistills.

Answer 1

P_H2O = P^o_H2O - P^o_mix

P_H2O = 760 Torr - (15 + 75) Torr = 670 Torr

g carvone / g water = ((P_carvone)(MW_carvone)) / ((P_H2O)(MW_H2O))

g carvnone / g water = ((15 Torr)(150g/mol)) / ((670 Torr)(18g/mol))

g carvone / g water = 2250 / 12060

g carvone / g water = 0.19 g carvone/1 g water

Question 2

Calculate how much limonene will distill out of every 1.0 g of water that codistills.

Answer 2

g limonene / g water = ((P_limonene)(MW_limonene)) / ((Pwater)(MWwater))

g limonene / g water = (75 Torr x 136 g/mol) / (670 Torr x 18 g/mol)

g limonene / g water = 10200 / 12060

g limonene / g water = 0.85 g limonene / 1g water

Question 3

What is always the maximum temperature needed for steam distillation? Why?

Answer 3

The maximum temperature is always less than the boiling point of water 100°C. Because the mixture is heterogeneous (immiscible), Raoult’s law (mole fractions) does not apply. Therefore the total pressure above the mixture is the sum of the individual equilibrium vapor pressure of each componenet (Dalton’s Law).

Question 4

Dalton’s Law

Answer 4

Dalton’s Law: P_mix = P_A + P_B + ….

For a mixture of carvone & limonene in water:

P_mix = (P_H2O + P_carvone + P_limonene) > P_H2O

Since equilibrium vapor pressure is inversely proportional to boiling point, the mixture has a lower boiling point than water

Question 5

Two components A + B are eluted with acetone on a TLC plate and found to have retention factors of 0.8 and 0.3, respectively. Which component is the most polar, why?

Answer 5

Assuming a normal-phase TLC plate where the stationary phase is polar (ex. Silica), component B would be more polar than component A. More polar compounds have a greater affinity for the TLC stationary phase, so they spend more time with that phase. Consequently, they move up the TLC plate more slowly, giving rise to smaller retention factors.