Chapter 18 – The Gas Laws
Section A – Gas Laws
Boyles Law
Charles Law
Gay – Lussac’s Law
Avagadro’s Law
Section B – The combined gas Law
P1 =
P2 =
V1 =
V2 =
T1 =
T2 =
*** Convert Celsius to Kelvin by
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Chapter 18 – The Gas Laws
Example 1
If a definite mass of gas occupies 500 cm3 at a pressure of 101,000 Pa and a temperature
of 27 oC, what is its volume in cm3 at s.t.p.? (Standard temperature = 0 oC, standard
pressure = 101,325 Pa)
Section C : The Kinetic theory of Gases
Assumptions





Gases are made up of particles whose ___________ are ______________ compared
to the _____________ between them.
There are no __________ or _____________ forces between particles.
Particles move in a ___________ motion, ____________ with each other and
container walls.
The average ____________ energy of the particles is _____________ to the Kelvin
temperature.
All collisions are perfectly _____________ ( speed of a particle before and after a
collision are the same)
Ideal gas
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Chapter 18 – The Gas Laws
Real gases deviate from ideal behaviour
Real gases deviate most from ideal behaviour when they are in conditions of __________
temperature and __________ pressures 

Particles now have less kinetic energy ( moving more slowly) and are pushed closer
together and do ____________ and ___________each other
The particles do have an ____________ _____________.
**** Gases which behave most like ideal gases will be the ones with the ____________
intermolecular forces between particles.
Think which of the following gases would be most like an ideal gas? – H2, HF, NH3 and F2
Section D: The equation of state for an ideal gas
P
V
n
R= 8.31JK-1mol-1
T
Example 1
How many moles of nitrogen gas are present in 200cm3 of the gas at 35oC and a pressure
of 200kPa? (The gas constant is 8.31JK-1 mol-1 )
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Chapter 18 – The Gas Laws
Section E: Mandatory experiment:
Determination of the relative molecular mass of a volatile liquid
using the ideal gas equation PV= nRT
A volatile liquid is a liquid with a ___________-__________point. Example:
A volatile liquid must be used in this experiment because it has a boiling point of less than
__________ which is the boiling point of water.
Procedure
1) Find the total ____________ of a dry clean conical flask, a circle of
aluminium foil and a rubber band.
2) Add about 3cm3 of a _____________ liquid into the flask.
3) Cover the flask with the aluminium foil using the rubber band and make
a small hole in the foil using a _____________.
4) _________________ the flask into a beaker of ___________water.
5) The liquid inside the flask will all _____________ and some of it will
escape through the hole in the foil. This will continue to happen
until_________________________________________________________________
____________________________________________________________________.
6) Remove the flask from the water when____________________________________.
7) Record the temperature of the hot water using a ___________________________
**( this is the temperature of the _______________________)
8) Record the atmospheric pressure in the room using a __________________
***( this is the pressure of the _____________________)
9) Allow the flask to cool. You will notice_____________________________________
____________________________________________________________________
10) Dry the outside of the flask, and then find the mass of the flask, rubber band and the
foil. ****( The mass of the vapour is now found by
___________________________________________________________________)
11) Remove the foil and band. Find the volume of the flask by filling it with water and
then measuring the volume of the water with a ________________ cylinder.
***** ( This is the volume of the __________________)
12) Calculate the value of the relative molecular mass of the gas using the equation
PV= nRT
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Chapter 18 – The Gas Laws
Example 1
In order to determine the relative molecular mass of a volatile liquid a conical flask was
filled with vapour of the liquid. The following results were obtained:
Atmospheric Pressure = 100000Pa , Volume of flask = 0.000315m3, Mass of vapour – 1.1g ,
temperature of the vapour = 300K
Calculate the relative molecular mass. ( To the nearest whole number)
(The gas constant is 8.31JK-1 mol-1 )
Answer:
Part 1 – List all quantities needed for the equation in the correct units
P = Pressure = 100000Pa , V = Volume = 0.000315m3, n = number of moles = ?, R = gas
constant = 8.31JK-1 mol-1 T = temperature = 300K
Part 2 – Sub into the equation
PV = nRT
(100000)(0.000315 ) = n (8.31) (300)
(100000)(0 .000315) = n
(8.31) (300)
0.0126
= n ( the number of moles)
Part 3 – Find the RMM
X Relative Molecular Mass
Mass g
Moles
÷ Relative Molecular Mass
0.0126 moles x RMM = 1.1 g
RMM = 1.1/ 0.0126
RMM = 87.3016
To the nearest whole number – RMM = 87
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Chapter 18 – The Gas Laws
Check your learning of
Gas Laws
Green = I know this already
Orange = I am not sure – have to revise this
Red = I don’t know this yet – have to learn it
Can you
Green
Orange
Red
State Boyle’s law.
State Charles’s law.
State Gay-Lussac’s law of combining volumes
(higher level only)
State Avogadro’s law.( higher level only)
Know that the combined gas law is used to calculate
changes in temperature , pressure or volume of a gas and
know how to use it
All of the below are Higher level only :
Be able to explain what the kinetic theory of gases is about
Know what an ideal gas is
Know how real gases deviate from ideal behaviour
Equation of state for an ideal gas:
PV = nRT (units: Pa, m3, K).
Know how to do calculations to find any missing part
Find the Relative Molecular mass of any gas ( once the
mass is given)
Using PV = nRT (units: Pa, m3, K).
Mandatory experiment – to calculate the relative molecular
mass of a volatile liquid using the ideal gas equation.
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