Chapter 4

Question 1

HETP Equation

Answer 1

HETP = height(h) / theoretical plates (N)

Question 2

A binary mixture of benzene and toluene is injected into a GC packed column with a Carbowax stationary phase. Benzene and toluene are found to have retention times of 2 and 5 min, respectively. Which componenet shows a greater affiinity for the stationary phase? Why?

Answer 2

Toluene has the longer retention time, so it must have a greater affinity for the stationary pase (larger partition coefficient, K_c). We can assume both compounds have similar polarities since they have very similar structures. Toluene, however, has a higher mass than benzene, which means it makes more attractive Van der Waals forces in the liquid phase and therefore has a lower equilibrium vapor pressure (i.e. higher boiling point). Having a lower equilibrium vapor pressure means it will have a lower concentration in the mobie phase than benzene, or, conversely a higher concentration in the stationary phase (i.e. a larger K_c).