Gas Laws
Chemistry 9(A)
Gas Laws
Learning Objectives
• Describe the relationships between volume, pressure,
number of moles, and temperature for an ideal gas
• Perform calculations that predict values for variables
using
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–
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–
–
Boyle’s law
Charles’ law
Avogadro’s law
Ideal gas law
Dalton’s law of partial pressure
Variables Affecting Gases
• Gases can be characterized by their
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–
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Volume ( V )
Temperature ( T )
Number of moles ( n )
Pressure ( P )
Gas Relationships
• Gas law – law which describes the relationship between
two or more variables of a gas
• Variables may be directly or inversely related
– Direct relationship – values of variables increase or decrease
together
– Inverse relationship – as the value one variable goes up, the value
of the other variable goes down
• Variables not included in the gas law remain constant
Charles’ Law
• Charles’ law describes the relationship between
temperature and volume, when the pressure and amount
of gas are constant
• Volume and temperature are directly proportional on the
Kelvin scale
Charles’ Law
• Temperature is directly
proportional to kinetic
energy
– Kinetic energy – energy
of motion
– At higher temperatures,
gas particles
• Have more kinetic energy
• Move more
• Spread out
Ex) Volume of a gas
when T1 < T2
Temperature Scales
Celsius vs. Kelvin scale
• Must use kelvin in
Charles’ law calculations
• Celsius to Kelvin scale
Ex) 25 ∘C + 273 = 298 K
Charles’ Law Calculation
Ex) A 2.4 L volume of gas is heated from 298 K to 405 K. What
new volume does it occupy?
1.
2.
Write unknown and givens
3.
4.
Convert units and find
intermediates, if needed
Plug in and solve
5.
Make sure the answer is reasonable
Identify the formula and
rearrange, if needed
Boyle’s Law
• Boyle’s law describes relationship between pressure and
volume when the temperature and amount of gas remain
constant
• Pressure and volume are inversely proportional
Pressure Units
Units of pressure
• Atmospheres (atm)
• Millimeters of mercury (mmHg)
• Kilopascals (kPa)
Equivalent amounts
• 1 atm = 760 mmHg = 101.3 kPa
Ex) How many kilopascals are equal to 5.00 atm?
Boyle’s Law Calculation
Ex) The initial pressure of a 3.0 L sample of gas was 2.5 atm. At
what pressure will the volume of the gas expand to 5.0 L?
1.
2.
Write unknown and givens
Identify the formula and
rearrange, if needed
3.
4.
Convert units and find
intermediates, if needed
Plug in and solve
5.
Make sure the answer is reasonable
Avogadro's Law
• Avogadro’s law describes the relationship between number
of moles and volume, when temperature and pressure
remain constant
• Number of moles of gas are directly proportional to
volume
Avogadro’s Law Calculation
Ex) How many moles of carbon monoxide gas are present in a
9.6 L sample if 4.2 moles were contained in 5.0 L sample?
1.
2.
Write unknown and givens
Identify the formula and
rearrange, if needed
3.
4.
Convert units and find
intermediates, if needed
Plug in and solve
5.
Make sure the answer is reasonable
Ideal Gas Law
• Combining Charles’, Boyle’s, and Avogadro’s laws gives the
following equation:
• A gas always has the same relationships between its
variables
– Charles –– volume and temperature are directly proportional
– Boyle –– pressure and volume are indirectly proportional
– Avogadro –– number of moles and volume are directly
proportional
• R is the ideal gas constant
Ideal Gas Law
• Ideal gas law describes the relationship between all four
variables: temperature, volume, number of moles, and
pressure
– Potential values for R depending on the pressure unit used
• 0.0821 L ·atm = 8.31 L · kPa = 62.4 L · mmHg
mol ·K
mol ·K
mol ·K
Ideal Gas Law Calculation
Ex) What is the volume of a 1.42 mol sample of O2 gas at 25 °C
and 1.25 atm of pressure?
1.
Write unknown and givens
3.
Convert units and find
intermediates, if needed
25 °C + 273 = 298 K
4. Plug in and solve
1.
Identify the formula and
rearrange, if needed
5.
Make sure the answer is reasonable
Dalton’s Law of Partial Pressure
• Dalton’s law of partial pressure describes the relationships
in a mixture of gases
• Total pressure of a gas mixture is equal to the sum of the
partial pressures of the individual gases
Dalton’s Law Calculation
Ex) A mixture of gases containing methane, CH4, ethane, C2H6, and
propane, C3H8, gases has a total pressure of 975 mmHg. If the partial
pressures of CH4 and C2H6 are 235 mmHg and 450mmHg, respectively, what
is the partial pressure of C3H8 ?
1.
2.
Write unknown and givens
Identify the formula and
rearrange, if needed
3.
4.
Convert units and find intermediates,
if needed
Plug in and solve
5.
Make sure the answer is reasonable
Gas Laws
Learning Objectives
• Describe the relationships between volume, pressure,
number of moles, and temperature for an ideal gas
• Perform calculations that predict values for variables
using
–
–
–
–
–
Boyle’s law
Charles’ law
Avogadro’s law
Ideal gas law
Dalton’s law of partial pressure