Gas Law Notes
Basic gas law problems

First identify the type of problem by asking: is the problem single state or is
there a change occurring?
o If single state then use PV=nRT
o If a change is occurring then derive
PV=nRT problems
 Identify each variable by its units
o (P) can be in atm or kPa
o (V) must be in L
o (n) is in moles
o (R) is determined by the pressure in the problem
o (T) must be in Kelvin
Derived problems
 Identify the variables that are changing
 Underline them within PV=nRT
 Put all variables on the same side
 Set them equal to one another
 Additional things to remember
o T must be in Kelvin ALWAYS
o Units on other variables just need to match
Dalton’s Law of Partial Pressures
The total pressure in the system is equal to the sum of all partial pressures
PTotal = Px + Py + Pz +…
Vapor pressure represents the partial pressure of water
PVap = PH2O
PV = nRT
This will allow you to convert between the pressure and moles of different things:
 P and n for the entire gas
 P and n for any one of the gasses in the mixture
Setup for Partial Pressures
(layout your variables like this)
PT =
Px =
PY =


nT =
nX =
nY =
T
V
Work down pressure or mole columns using PT = Px + Py … and nT = nx + ny …
If you have temp or volume use PV=nRT to to work across the row
o Plugging in PT with V or T will give you nT or vice versa
 Same for any PP and individual gas
o If you’re looking for T or V, make sure you use the same P and n
 Ex: Px and nx or PT and nT
 Do not mix: PT and nX is not allowed
Kinetic molecular theory
(The logic behind gas laws)
There are two approaches to solving these problems:
1. Picture it:
a. Think of a container of gas as you within a phone booth filled with
bouncy balls
b. The impact of the bouncy balls on you is what causes pressure
c. Consider the changes being made to the container or the bouncy balls
and the effect they will have
d. Remember that heating up the gas increases the speed of the bouncy
balls
2. The algebraic approach using PV=nRT:
a. Variables on the same side of the equation, such as P and V, are
inversely proportional
i. If P increases then V must decrease by the same magnitude for
that side to remain equal to the constant nRT on the other
b. Variables on opposite sides, such as P and T are directly proportional
i. If T doubles then P must as well in order to keep both sides
equal