Unit 10: Gases
Chemistry I
Mr. Patel
SWHS
Topic Outline
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MUST have a scientific calculator (not graphing)!!
Properties of Gases (13.1, 14.1)
Kinetic Molecular Theory (13.1)
Temperature Conversions (14.1)
Pressure Conversions (14.1)
Empirical Gas Laws (14.2)
Ideal Gases (14.3)
Nature and Properties of Gases
• Gases are easily compressible
– Flow easily (fluids)
– Assume the shape and volume of container
• Gases are described by four variables:
– Amount of gas (n)
– Volume (V)
– Pressure (P)
– Temperature (T)
• Kinetic theory – all particles are in constant motion …one exception
Kinetic Molecular Theory
1. Particles have insignificant volume
– Large distance between particles
– No attractive or repulsive forces
2. Motion is rapid and random
– Travel in straight line path until collisions
3. Collisions are completely elastic
– KE is transferred without loss
Temperature
• When heating a gas, most energy goes into
motion
• Average KE is related to temperature (K)
• All particles same average KE at room temp
• All motion stops at absolute zero
– 0 K or -273⁰C
Temperature Conversions
• Celsius to Kelvin:
K = 273 + ⁰C
• Kelvin to Celsius:
⁰C = K - 273
Ex: Convert 924.8 K to Celsius.
Ex: The hottest planet in the solar system
measured a temperature of 480 K on a certain
day. What is this temperature in Celsius?
Ex: The Rhinovirus reproduces especially well at
nostril temperatures of 32.8⁰C. What is this
temperature in Kelvin?
Gas Pressure
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Force exerted by gas per unit surface area
Result of billions of simultaneous collisions
Ex: Helium balloon
Vacuum – No pressure
– no gas particles
• Barometer – device used to
measure atmospheric
pressure
Pressure Conversions
• Standard Temperature and Pressure (STP)
– 0⁰C = 273 K
– 1 atm
• 1 atm = 760 mm Hg = 760 torr = 101.3 kPa (SI)
• These are conversion factors
Ex: Convert 25.3 atm to mm Hg.
Ex: The air pressure for a certain tire is 109 kPa.
What is this pressure in atmospheres?
Ex: The weather news gives the atmospheric
pressure as 1.07 atm. What is this
atmospheric pressure in mm Hg?
Practice
1. An experiment at Sandia National Labs in
New Mexico is performed at 758.7 torr. What
is this pressure in atm?
2. A bag of potato chips is sealed in a factory
near sea level. The atmospheric pressure at
the factory is 761.3 mm Hg. The pressure
inside the bag is the same. What is the
pressure inside the bag of potato chips in
kPa?
Practice Solutions
1. Answer: 0.998 atm
2. Answer: 1.01 kPa
The Empirical Gas Laws
• There are four empirical gas laws
• These laws relate the gas variables
• Empirical – based on observation
Boyle’s Law
• Relates
Pressure and Volume
• Law: P1 V1 = P2 V2
• Inverse relationship
• If V
then P
Ex: If the volume of the lungs is 2.400 L during
exhalation and the pressure is 101.70 KPa, and
the pressure during inhalation is 101.01 KPa,
what is the volume of the lungs during
inhalation?
Charles’s Law
• Relates
Temp and Volume
• Law:
• Direct relationship
• If T
then V
Ex: If the volume of a gas at 25⁰C is 25.5 L, what
is the temperature when the volume is 75.0 L?
Gay-Lussac’s Law
• Relates
Pressure and Temp
• Law:
• Direct relationship
• If T
then P
Ex: If the pressure of a gas at 75⁰C is 12.5atm,
what is the temperature when the pressure is
3.75 atm?
Combined Gas Law
• Combination of the previous empirical laws
Avogadro’s Law
• Relates
Moles and Volume
• Law:
𝑽𝟏 𝑽𝟐
=
𝒏𝟏 𝒏𝟐
• Direct relationship
• If n
then V
Ex: If the moles of gas is 3.75 mol N2 is used to
inflate a balloon to 0.250L, what is the volume
when the moles of gas are tripled?
The Empirical Laws
• The previous laws are useful when only two
variables are changing…everything else is
constant.
• They are laws for specific scenarios.
• It is possible to use the laws to create a law
that is general in nature – not specific.
Ideal Gas Law
• Combines the previous observations.
• One of the most fundamental and important
laws in chemistry.
• Describes a SINGLE gas.
PRESSURE
𝑷𝑽 = 𝒏𝑹𝑻
VOLUME
MOLES
TEMPERATURE
GAS CONSTANT
Ideal Gases
• This is for ideal gases…not real gases.
• Many real gases behave as ideal gases.
• An ideal gas completely follows all the tenants
of the Kinetic Molecular Theory.
Gas Constant – R
• This is a universal constant.
• It is important to memorize the value of this
number with the CORRECT units.
• The value changes based on the units.
atm∙L
R = 0.0821 mol∙K
Implications of R on PV=nRT
• Previous laws – we could use any units as long
as we were consistent.
• The units used in the ideal gas law are
determined by our choice of R.
• If R = 0.0821 atm∙L/mol∙K is used:
– P must be in atm
– V must be in L
– T must be in K
Tips on Solving Gas Law Problems
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Write down all the variables that are known.
Determine which variable you are looking for.
Convert all variables to the correct units.
Write down the correct law/equation.
Rearrange equation.
Plug in values and solve.
See if your answer makes sense.
Ex: The gas in a spray can has a pressure of 103
kPa at 25 ⁰C. The can is heated to 928 ⁰C.
What is the resulting pressure?
Ex: A balloon has a volume of 30.0L at 313K and
a pressure of 1147.9 mmHg. What is the
volume at 0 ⁰C and 1 atm?
Ex: The temperature of a gas is 300.K and the
volume is 821 L. The temperature of the gas is
raised by 35.0 K. What is the new volume?
Ex: The volume of a sealed, rigid cylinder is
20.0L. The pressure in the cylinder is 985 torr
at 31 ⁰C. What is the effect when the
temperature is lowered to 3.1 ⁰C?
Ex: An isothermal system contains CO2 gas at
900 K with a pressure of 5.31 kPa. What is the
volume of the gas?
Ex: At a pressure of 1.00 atm, one mole of a gas
is at 0⁰C. What volume does it occupy?
Ex: A container with a volume of 3000. mL holds
32.0 g CH4 gas at 25.0⁰C. What is the pressure
within the container?
Ex: In a 3.05L bottle, CO2 gas is heated to 298K
at a pressure of 225.1 kPa. How many grams
of carbon dioxide gas are in the bottle?