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Chemistry – Dr. May
Two Kinds of Gas Law Problems
The Combined Gas Law includes Boyle’s and Charles Law and can be used for
any gas problem where conditions change.
P1V1 ‗ P2V2
T1
T2
P = pressure, and calculations can be done when P is in millimeters of Hg (mm),
kilopascals (kPa), or atmospheres (atm) as long as both pressures are in the same units.
Standard pressure is 760 mm, 101.32 kPa, or 1 atm.
T = temperature, and calculations can be done only when T is in Kelvins. Standard
temperature is 273 K or 0oC.
V = volume of gas and calculations can be done when V is in any unit as long as both
volumes are in the same units. The most common units are liters (L), milliliters (ml), and
cubic centimeters (cm3). One mole of gas at STP (0oC and 1 atm) occupies 22.4 liters.
Sample Problem:
A balloon in a hot vacuum oven at 60 oC and a pressure of 40 kPa
has a volume of 60 liters. What would the volume be when the balloon is
taken to standard conditions (STP)?
P1V1 ‗ P2V2
T1
T2
P1
V1
T1
P2
V2
T2
=
=
=
=
=
=
______________
‗ ____________
?
V2 =
1
The Ideal Gas Law is used to determine the properties of a volume of gas under
static conditions.
PV = nRT
We can express the equation as a function of each of the variables by solving for
them algebraically. These equations can then be used to solve for any of the four
variables when the other three are known and expressed in the proper units.
P ‗ nRT
V
V ‗ nRT
P
T ‗ PV
nR
n
‗ PV
RT
It is very important that the numbers used in calculations in the Ideal Gas Law
have the proper units.





R is the universal gas constant and is always 0.0821 when P is in atmospheres and
is always 8.3 when P is in kilopascals.
V is always in liters
P is in atmospheres or kilopascals ( 1 atm = 760 mm Hg or 1 atm = 101.32 kPa)
and must be matched up with the proper universal gas constant. (With
atmospheres use R = 0.0821 and with kilopascals use R = 8.3)
n is always in moles
T is always in Kelvins (To get Kelvins add 273 to oC)
Anytime moles are involved, we must remember the following relationship:
moles ‗ grams
MW
or
grams = moles x MW
Sample Problem:
What is the volume of a vial containing 42 grams of nitrogen at a pressure of
290 kPa and a temperature of 29 oC.
n
R
T
P
V
=
=
=
=
= ?
V
V
‗
‗
nRT
P
______________________
2
V =