Chem 1310 I
T. F. Block
Fall, 2004
Quiz 6 Key
Your name_____________________
Quizzes are not graded. They are intended to illustrate the kinds of questions which may appear on exams.
Questions on this quiz deal with interactions between particles (latter sections of Chapter 5, Gas Laws, and
early sections of Chapter 6, Condensed Phases and Equilibrium).
1. A gas has the empirical formula, CH2. It has a density of 1.70 g/L at
25.0 ˚C and 740.0 torr. What is the molecular formula of the gas?
a. C2H4
b. C3H6
c. C3H8
d. C4H10
(0.974 atm)(1 L) = n (0.08206 L atm mol–1K–1)(298.2 K)
n = 0.0398 mol
(1.70 g/0.0398 mol) = 42.7 g/mol
2. Which gas would probably have the smallest molar volume at STP?
a. Ne
b. C3H6
c. N2H4
d. H2Se
Hydrazine, N2H4, would exhibit hydrogen bonding.
Hydrogen selenide is polar but doesn’t exhibit hydrogen bonding.
Propane, C3H6, and neon are non-polar.
3. What is the partial pressure of nitrogen gas in a sample of N2
collected over water at 23.0 ˚C if the barometric pressure is 746.2 torr
and the height of the water column inside the eudiometer tube is 145.3
mm higher than the water level outside the tube? The vapor pressure of
water at 23.0 ˚C is 21.1 torr and mercury is 13.6 times more dense than
water.
a. 688.5 torr
b. 714.4 torr
145.3 mm H2O corresponds to 10.7 mm Hg
Pnitrogen = 746.2 torr – 21.1 torr – 10.5 torr
c. 599.3 torr
d. 760 torr
4. Which of the substances identified below should have the highest
boiling point?
a. C2H6
b. C7H14
c. C10H22
d. C18H38
These are all non–polar substances; interactions between molecules are governed by van
der Waals’ forces and become stronger with increasing molar mass (number of
electrons).
5. Which of the substances identified below should have the highest
boiling point?
a. HF
b. HCl
c. HBr
d. HI
These are polar substances that also differ significantly in molar mass (number of
electrons). In HCl, HBr, and HI, van der Waals’ forces are important and the boiling
point increases from HCl to HBr to HI. However, in HF, there is a high negative charge
density on the F (fluorine is very good at attracting shared electrons and it is a very small
atom). Therefore, hydrogen bonds form between the (somewhat) positive H in one
molecule and the (somewhat) negative F in another. The need to break theses extra bonds
before the individual molecules can enter the gas phase means that a higher temperature
must be reached before HF will boil.
6. Which of the substances identified below should have the highest
boiling point?
a. H2O
lowest C3H8 <
non-polar
b. C3H8
H2S
polar
<
c. LiCl
H2O
<
hydrogen bonding
d. H2S
LiCl
ionic
highest
7. Refer to the phase diagram sketched below. What transition will
occur when the temperature is raised along line AB?
a. freezing (liquid→solid)
b. deposition (gas→solid)
c. condensation (gas→liquid)
d. melting (solid→liquid)