Gas Laws

1
Chapter 12

• Gas Laws

2

• Introduction
• A. Gases are affected by
• 1) volume
• 2) temperature
• 3) pressure
• 4) the number of molecules
• B. Changing any one of these affects the others

3

• C. What would you expect to happen
• 1) to volume when a gas is heated?
• 2) to pressure when a gas is heated?
• 3) to pressure when volume gets smaller?
• 4) to temperature when volume shrinks?

4

• D. What factors are involved in each?
• a) inflating a tire with air
• b) heating an aerosol can
• c) warming up a closed bottle of soda
• d) cool liquid felt coming from a spray can

5

• E. Gas behaviors assume that
• 1) gases are tiny particles
• 2) there is a huge distance between molecules
• 3) gas particles do not attract each other
• 4) they travel in a straight line until they
  hit another molecule or the wall
• 5) no energy is lost after any collisions
• 6) the average kinetic energy is the same for
  any gas at the same temperature (but their speed
  isnt the same)

6

• II. The Gas Law Formulas
• A. The variables involved are

variable units
Temperature (T) Kelvin oC 273
Volume (V) mL , L
Pressure (P) mm Hg, torr, atm, kPa
Moles (n) moles
Conversions for pressure units 1 mm Hg 1
torr 760 torr 1 atm 101.3 kPa 1 atm
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• B. Combined gas law

C. If any variable is constant, dont use it
if temperature is constant
(Boyles Law)
if volume is constant
(Gay-Lussacs Law)
if pressure is constant
(Charles Law)
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• D. Daltons Law of Partial Pressures
• Ptotal P1 P2 P3 ...

The sum of individual partial pressures of each
gas equals the total pressure of the mixture
E. The Ideal Gas Law PV nRT
P pressure (atm) V volume (liters) n
moles R 0.0821 (L-atm/K-mol) T Kelvin
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Collecting a Gas Sample Over Water(Application
of Daltons Law)

• The pressure in the collection container is equal
  to the atmospheric pressure.
• The pressure of the gas collected plus the
  pressure of water vapor at the collection
  temperature is equal to the atmospheric pressure.

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Oxygen collected over water.
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A sample of O2 was collected over water in a
bottle at a temperature of 25oC when the
atmospheric pressure was 760 torr. What is the
pressure of the O2 alone? The vapor pressure of
water at 25oC is 23.8 torr.
(The pressure of the gas alone is sometimes
phrased as the pressure of the dry gas)
Ptotal 760 torr PO2 PH2O
PO2
PO2 760 - PH2O
PO2 760 - 23.8
736 torr
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A sample of nitrogen was collected over water and
occupies 300. ml at 23oC and 750 torr. If the
pressure is increased to 760 torr and the
temperature is decreased to 0oC, what would be
the new volume of the dry nitrogen? Vapor
pressure of H20 _at_ 23oC is 21.0 torr.
P1 729 torr P2 760 torr V1 300. mL V2
? T1 296 K T2 273 K
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A sample of nitrogen was collected over water and
occupies 300. ml at 23oC and 750 torr. If the
pressure is increased to 760 torr and the
temperature is decreased to 0oC, what would be
the new volume of the dry nitrogen? Vapor
pressure of H20 _at_ 23oC is 21.0 torr.

• Step 2. Write and solve the equation for the
  unknown V2.

14
A sample of nitrogen was collected over water and
occupies 300. ml at 23oC and 750 torr. If the
pressure is increased to 760 torr and the
temperature is decreased to 0oC, what would be
the new volume of the dry nitrogen? Vapor
pressure of H20 _at_ 23oC is 21.0 torr.
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• III. The Ideal Gas Law
• A. ideal gases are defined as gases
• 1) whose volume is insignificant
• 2) that have no attractions to other gas
  molecules
• B. PV nRT involves moles of gas and moles
  g/M
• C. By rearranging this equation, you can find
• 1) g
• 2) M
• 3) density

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• IV. Gas Law Stoichiometry
• A. Avogadros Law
• - equal volumes of different gases at the
  same temperature and pressure have the same
  number of molecules
• B. If gases are at the same temperature and
  pressure, then the coefficients of a balanced
  equation represent VOLUME of gas (liters,
  milliliters)
• ex How many liters of nitrogen gas are required
  to completely react with 1.00 liter of hydrogen
  gas to form ammonia if both gases are at the same
  temperature and pressure?
• ANS 0.333 liters

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• C. STP standard temperature and pressure
• 1) standard temperature 273 K 0 oC
• 2) standard pressure 1 atm 760 torr
• D. Molar Volume the volume of any gas measured
  at STP occupies 22.4 L
• At conditions other than STP, the PVnRT
  equation would be needed to find volume.

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• E. Gas law stoichiometry problems
• 1) PVnRT is used to calculate moles of gas
• 2) stoichiometry is used to calculate moles or
  grams of solids
• F. Which equation do you use first?
• 1) it depends
• 2) if grams of solid are given, start with
  stoichiometry
• 3) if grams of solid are unknown, start with gas
  law

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• F. Examples
• 1) What volume of oxygen gas can be formed at
  0.970 atm and 20.0 C from the decomposition of
  1.00 g of KClO3?
• 2 KClO3 ? 2 KCl 3 O2

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• F. Examples
• 2) What mass of magnesium will produce 250. mL
  of hydrogen gas measured at 20 C and 755 torr?
• Mg 2 HCl ? MgCl2 H2

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• F. Examples
• 3) What mass of nitrogen gas measured at STP
  will produce 28.0 g of ammonia at STP?
• N2 3 H2 ? 2 NH3

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• V. Grahams Law of Effusion
• A. All gases have the same average kinetic
  energy at the same temperature
• B. The velocity of gas molecules is inversely
  proportional to their molecular weights

C. Example An unknown gas effuses at a rate of
31.50 ml/min when oxygen effuses at a rate of
30.50 ml/min under the same conditions. Which of
these gases could be the unknown?
CH4 CO NO CO2 NO2
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D. Example An unknown gas effuses at a rate
1.21x the rate of CO2. What is the molecular
weight of the unknown gas?
Ans 44 g/mol
Chapter 12 . even numbered problems 26-64 ..
68, 70, 78, 88, 90, 92, 96