30 January 2012

Objective: You will be able to:
 review and discuss answers to the midterm
exam.
Agenda
Objective and Agenda
 Opportunities
 Midterm exam answers and discussion
Homework: Read p. 174-194
Tomorrow: Gases!

Field Trip Opportunity
Novartis is a biotech company in
Cambridge that aims to discover, develop
and successfully market innovative
products to prevent and cure diseases, to
ease suffering and to enhance the quality
of life.
 Tour, learn about what scientists at
Novartis does, learn about biotech careers
 Wednesday, February 15 periods 1-4

Remember this?
Harvard Museum of Natural History
Exhibit: The Language of Color
 Why do we see the colors we see?
 Why does a blue butterfly look blue?
 Why does a blue fish look blue?

Exam Review/Answers

Please be sure to return all parts of your
midterm.
31 January 2012

Objective: You will be able to:
 describe and convert to standard units
for temperature and pressure
 use the Gas Laws to make calculations
for pressure, temperature, volume and
number of moles of a gas.
Gases
Gases Topics
1.
2.
3.
4.
5.
6.
7.
Definitions
The Gas Laws
The Ideal Gas Equation
Gas Mixtures and Partial Pressures
Kinetic Molecular Theory of Gases
Effusion and Diffusion
Real Gases Deviate from Ideal Behavior
Definitions
Pressure: the force that acts on a given area.
 Gases exert a pressure on any surface they
contact.
http://www.mhhe.com/physsci/chemistry/essen
tialchemistry/flash/gasesv6.swf
 Standard pressure (at sea level):

1.01x105 Pascals = 101 kPa = 1 atm = 760 mm Hg = 760 torr


Standard Temperature: 0oC = 273 Kelvin
Standard Temperature and Pressure =
1 atm and 0oC.
The Gas Laws

Relate the pressure (P, atm), volume (V, liters)
and temperature (T, Kelvin) of a sample of gas
P1V1 P2V2

T1
T2

http://phet.colorado.edu/en/simulation/gasproperties
If you hold T constant…
Boyle’s Law: P1V1=P2V2
Boyle’s Law

If the pressure of a fixed amount of gas
doubles at a constant temperature, what
happens to its volume?
If you hold P constant…

Charles’ Law: V1
V2

T1 T2
Charles’ Law

When the temperature of a fixed amount
of gas doubles from 20oC to 40oC, what
happens to the volume at constant
pressure?
Avogadro’s Hypothesis


Equal volumes of gases under the same
conditions of temperature and pressure contain
equal numbers of molecules
a mole of gas at STP has a volume of 22.4 L
Avogadro’s Law

The volume of a gas at constant
temperature and pressure is directly
proportional to the number of moles of the
gas
V1 V2

n1 n2

The coefficients in a balanced equation can
be taken as ratios of moles or liters at
constant T and P
Example 1

How many liters of water vapor can be
obtained from the complete combustion of
24 liters of methane gas? Assume
constant temperature and pressure and
water in the gas phase.
Example 2

A weather balloon on the ground contains
25.8 L of He at 29.0oC and 741 torr. What
is the volume of the balloon when it rises
to an altitude where the temperature is
11.0oC and the pressure is 535 torr?
Problem 1

The gas pressure in an aerosol can is 1.5
atm at 25oC. Assuming that the gas inside
is an ideal gas, what would the pressure be
if the can were heated to 450oC.
Problem 2

A large natural gas storage tank is
arranged so that the pressure is
maintained at 2.20 atm. On a cold day in
December when the temperature is -15oC
(4oF), the volume of gas in the tank is
3.25x103 m3. What is the volume of the
same quantity of gas on a warm July day
when the temperature is 31oC (88oF)?
The Ideal Gas Equation

Combines Boyle’s, Charles’ and Avogadro’s
Laws
PV=nRT
P=
V=
n=
T=
R=

ideal gas: a hypothetical gas whose
pressure, volume and temperature
behavior is completely described by the
ideal gas equation
 no gas is an ideal gas
 all gases are “real” gases
 but most gases, above 25oC and below
1 atm, behave like ideal gases

p. 216 #16, 19, 21, 23, 31, 33
1 February 2012
Take Out Homework
 Objective: You will be able to:
 use the ideal gas equation to solve for the
pressure, volume, temperature or number
of moles of a gas.
 Homework Quiz: The volume of a gas is
5.80 L, measured at 1.00 atm. What is the
pressure of the gas in mmHg if the volume
is changed to 9.65 L? Temperature remains
constant.

Agenda
Homework Quiz
II. Homework answers
III. Using the Ideal Gas Equation
IV. Partial Pressure
V. Molar volume of gas lab introduction
Homework: p. 217 #35, 37, 39, 41, 51, 53 –
Mon., read lab and do pre-lab questions
in lab notebook - tomorrow
I.
Using the Ideal Gas Equation
Example 1
Gasoline is a mixture of many hydrocarbon
compounds but its chemical formula can
be approximated as C8H18. How many
liters of carbon dioxide gas are formed at
25.0oC and 712 torr when 1.00 gallon of
liquid gasoline is burned in excess air?
Liquid gasoline has a density of 0.690
g/mL. One gallon = 3.80 L
Problem 3

Calcium carbonate, CaCO3(s), decomposes
upon heating to give CaO(s) and CO2(g).
A sample of CaCO3 is decomposed, and
the carbon dioxide is collected in a 250mL flask. After the decomposition is
complete, the gas has a pressure of 1.3 atm
at a temperature of 31oC. How many
moles of CO2 were generated?
Problem 4

Tennis balls are usually filled with air or
N2 gas to a pressure above atmospheric
pressure to increase their “bounce.” If a
particular tennis ball has a volume of 144
cm3 and contains 0.33 g of N2 gas, what is
the pressure inside the ball at 24oC?
6 February 2012
Objective: You will be able to:
 calculate the density of a gas
 describe kinetic molecular theory
 define effusion and diffusion
 Homework Quiz: Methane, CH4, the
principal component of natural gas, is used
for heating and cooking. If 10.0 moles of
methane are reacted, what is the volume of
CO2, in liters, produced at 23.0oC and 0.985
atm?

Agenda
Homework Quiz
II. Homework answers
III. Density calculations
IV. Partial pressures
V. Kinetic-Molecular theory
VI. Effusion and Diffusion
Homework: p. 218 #63, 65, 67, 69, 71, 93, 95
I.
Example
What is the density of sulfur dioxide gas at
35oC and 1270 torr?
Problem 5

What is the density of carbon
tetrachloride vapor at 714 torr and 125oC?
Problem 6
A series of measurements are made to determine
the molar mass of an unknown gas. First, a
large flask is evacuated and found to weight
134.567 g. It is then filled with the gas to a
pressure of 735 torr at 31oC and reweighed. Its
mass is now 137.456 g. Finally, the flask is filled
with water at 31oC and found to weigh 1067.9 g.
Calculate the molar mass of the unknown gas.
(The density of the water at this temperature is
0.997 g/mL)
Gas Mixtures and Partial Pressures
partial pressure: the pressure exerted
by an individual gas in a mixture of gases
 Dalton’s Law of Partial Pressures:
the total pressure of a mixture of gases is
the sum of the pressures that each gas
would exert if it were present alone.
Ptotal = P1 + P2 + P3 + …

mole fraction: the ratio of moles of one gas
to the total moles of gas in a mixture
X1 = n1/nt
Each gas in a mixture behaves
independently of all the other gases in the
mixture and obeys the ideal gas law.

Example 1
A mixture of 9.00 g oxygen, 18.0 g argon
and 25.0 g of carbon dioxide exerts a
pressure of 2.54 atm. What is the partial
pressure of argon in the mixture?
2 February 2012
Objective: You will be able to:
 determine the molar volume of a gas by
generating and measuring hydrogen gas,
and using the gas laws.
 Do now: Grab a pair of goggles

Agenda
Objective
II. Pre-lab questions
III. Lab directions
IV. Do the lab!
V. Collect and analyze data
VI. Post lab questions
Homework: Book work assigned yesterday
Lab notebook: due Monday
I.
Molar Volume of a Gas Lab
Weight your 3 cm piece of magnesium
instead of calculating the mass.
 Follow the directions carefully!
 Leave the HCl at the front – bring your
graduated cylinder.
 Use the 1000 mL graduated cylinder for
step 15. We only have one. Take turns.
 Barometric pressure = 30.03 in Hg now

p. 7 #7:

graduated cylinder eudiometer
Your lab notebook
1.
2.
3.
4.
5.
6.
7.
8.
Name, etc. on top.
Purpose
Pre-lab questions
Procedure summary
Data organized into a table or along with
procedure
Calculations, including units and labels
describing what the calculation is!
Answer to post lab questions
One more source of error and specifically how
it affects your data. How would you correct it?
Problem 7

A gaseous mixture made from 6.00 g O2
and 9.00 g CH4 is placed in a 15.0 L vessel
at 0oC. What is the partial pressure of
each gas, and what is the total pressure in
the vessel?
7 February 2012
Objective: You will be able to:
 describe the Kinetic Molecular theory of
gases
 describe effusion and diffusion and calculate
effusion rate
 Homework Quiz:
a. What is the mass of the solid NH4Cl formed
when 73.0 g of NH3 gas are mixed with an
equal mass of HCl gas?
b. What is the volume of the gas remaining,
measured at 14.0oC and 752 mmHg? Which
gas is it (NH3 or HCl)?


Homework p. 219 #76, 93, 101, 103, 105,
106, 132
Problem 8

What is the total pressure exerted by a
mixture of 2.00 g of H2 and 8.00 g of N2 at
273 K in a 10.0 L vessel?
Problem 9

a.
b.
A study of the effects of certain gases on
plant growth requires a synthetic atmosphere
composed of 1.5 mol percent CO2, 18.0 mol
percent O2 and 80.5 mol percent Ar.
Calculate the partial pressure of O2 in the
mixture if the total pressure of the
atmosphere is 745 torr.
If this atmosphere is to be held in a 121 L
space at 295 K, how many moles of O2 are
needed?
Kinetic Molecular Theory

1.
2.
3.
4.
5.
a model that explains the macroscopic behavior of
gases at the atomic and molecular level.
Gases consist of atoms or molecules in
continuous, random motion
The volume of the gas particles is negligible
relative to the volume of their container
Attractive and repulsive forces between gas
particles are negligible.
Collisions between gas particles and between gas
particles and the walls of their container are
perfectly elastic (no energy is lost)
The average kinetic energy of the particles is
proportional to the absolute temperature.
Problem 10

Use kinetic molecular theory to explain
why, when the temperature of a fixed
volume of gas increases, the pressure
increases.
Problem 11
A sample of O2 gas initially at STP is
compressed into a smaller volume at
constant temperature. What effect does this
change have on:
a. the average kinetic energy of O2 molecules?
b. the average speed of O2 molecules?
c. the total number of collisions of O2
molecules with the container walls in a
unit time?
d. the number of collisions of O2 molecules
with a unit area of container wall per unit
time?

8 February 2012
Objective: You will be able to:
 describe effusion and diffusion and
calculate effusion rate.
 Homework Quiz:
a. A mixture of gases contains 4.46 moles of
neon and 0.74 moles of argon. Calculate the
partial pressures of the gases if the total
pressure is 2.00 atm at a certain
temperature.
b. Calculate the volume occupied by the argon
gas at 0oC.

Agenda
Homework quiz
II. Homework answers
III. Effusion and Diffusion
IV. Diffusion demo and video
Homework: Gases problem set: Mon.
I.
Effusion and Diffusion
Effusion: the escape of gas molecules
through a tiny hole into an evacuated space.
 Diffusion: the spread of one substance
through space or through another
substance.
 Graham’s Law of Effusion: the effusion
rate of a gas is inversely proportional to the
square root of its molar mass.
 the equation relates the ratio of effusion
1
rates of two gases:
r1  M 2  2

 
r2  M 1 


http://www.youtube.com/watch?v=WAJ
Aslkwolk
Example
At a particular temperature and pressure,
neon gas effuses at a rate of 16.0 mol/s.
a. What is the rate at which argon effuses
under the same conditions?
b. Under a different set of conditions, 3.0
mol of argon effuse in 49.0 seconds.
How long will it take an equal amount of
helium to effuse?

9 February 2012


Objective: You will be able to practice gas
unit objectives and calculations.
Homework Quiz: A gas of unknown
molecular mass was allowed to effuse
through a small opening under 2.0 atm and
20oC. It required 105 s for 1.0 L of the gas to
effuse. Under identical conditions, it
required 31 s for the same amount of O2 gas
to effuse. Calculate the molar mass of the
unknown gas.
Problem 12

An unknown gas composed of
homonuclear diatomic molecules effuses
at a rate that is only 0.355 times that of O2
at the same temperature. Calculate the
molar mass of the unknown and identify
it.
Real Gases: Deviations from Ideal Behavior
Real gas particles have volume!
 Real gas particles have attractive and
repulsive forces between them!
 Especially at high pressure and low
temperature, real gases do not be have
ideally.
 Deviation from ideal behavior increases
with increasing complexity and mass of a
molecule of a gas.

Problem 13


Which of the noble gases will deviate most
from ideal behavior? Why?
Arrange the following in order of
increasing deviation from ideality:
H2O(g), CH4(g), Ne
Justify your answer
Problem Set Work Time
30 minutes: independently
 Then, with a partner
 Why?
 If you’ve never tried problems
independently before a test…
 The fewer brains, the more yours works

Homework
Problem Set: due Monday
 Gases Unit Test Tuesday
 10 MC
 1 multi-part free response question
 Review session: tomorrow 3:20-4:15 in
205


What weapon can you make from
potassium, nickel and iron?
14 February 2012
Take Out: Periodic table and calculator
 Objective: You will be able to:
 show what you know about gases!
