Review 10/26
Review 10/26
1. Which of the following compounds has the lowest
vapor pressure at 25°C?
2. The phase diagram of a substance is given below.
When the temperature is raised from 20°C to 45 °C at
0.8 atm pressure, what phases will be observed during
this temperature change?
A.
HO
B.
OH
C.
OH
CH3OH
OH
A. solid"liquid
D.
B. solid"gas
E.
OH
HO
OH
Solution: Given structural formula, to find the
compound with the lowest vapor pressure
1. At the same temperature, compound with the
strongest intermolecular forces has the lowest vapor
pressure.
2. All five choices have similar types of intermolecular
forces: the London dispersion forces, dipole-dipole
forces, and Hydrogen bonding. A has the most –OH
groups and the heaviest molecular weight, so A has the
strongest intermolecular forces. As a result, A has the
lowest vapor pressure.
! Answer: A
C. liquid"gas
D. solid"liquid"gas
E. gas only
Solution:
1. From the phase diagram, we see that w represents
solid phase, x represents liquid phase, y represents gas
phase, and z represents triple point.
2. At 0.8 atm and 20 ºC, the substance falls in the
region of x (liquid phase). At 0.8 atm and 45 ºC, the
substance falls in the region of y (gas phase).
Therefore, C is correct answer.
! Answer: C