GAUTENG DEPARTMENT OF EDUCATION SENIOR SECONDARY INTERVENTION PROGRAMME
PHYSICAL SCIENCES
Grade 11
SESSION 14
(LEARNER NOTES)
IDEAL GAS LAWS
Learner Note: The theory in this section is important, it relates to intermolecular forces
and the kinetic theory. Remember the factors that allow a gas to condensate and become
a liquid – for liquids the Gas Laws will not apply.
SECTION A: TYPICAL EXAM QUESTIONS
Question 1: 15 minutes (Adapted from DoE Exemplar 2007)
Most modern cars are equipped with airbags. The following reaction allows a gas to be
produced to fill the airbag.
2 NaN3 (s) → 2 Na (s) + 3 N2 (g)
(Remember to consider the units of the given information when doing this calculation!
Pay attention to the “case” of the symbols used in the equation – all marks will be
forfeited in the event of a wrong equation!)
1.1
Calculate the mass of N2 needed to inflate a sample airbag to a volume of 65 dm 3 at
25 C and 99,3 kPa. Assume the temperature remained constant.
(7)
1.2
Explain why nitrogen can / cannot be referred to as an ideal gas.
1.3
Why can we assume that nitrogen reacts as an ideal gas in the example given? (2)
1.4
Name two real gases that behave most like ideal gases at high pressures and low
temperature.
(2)
(5)
(16)
Learner Note: Emphasise the difference between real and ideal gas behaviour. Relate to
the gases that are close to ideal, understand why they behave like ideal gases
Page 1 of 6
GAUTENG DEPARTMENT OF EDUCATION SENIOR SECONDARY INTERVENTION PROGRAMME
PHYSICAL SCIENCES
Grade 11
SESSION 14
(LEARNER NOTES)
Question 2: 15 minutes
Learner Note: This can be a difficult example, relate to theories previously done. Use
General Gas Equation to show relationships, understand the use of this formula rather
than just see it as a tool to do a calculation
(Remember – convert molecules to mol. The general gas equation can be manipulated
to get the ratio. Hydrogen is close to an ideal gas but will still vary at low temperatures,
however the deviations are small).
9,54 x 1022 molecules of hydrogen gas are sealed in a container of which the volume can
change eg. syringe. The relationship between p, V and T is investigated at temperatures
ranging from - 20 C to 120 C.
(Remember the criteria for drawing a graph)
2.1
Draw a pV against T graph.
(5)
2.2
Calculate the ratio pV/ T for the investigation at 0 C.
(6)
2.3
When the temperature is lowered to 35 K the hydrogen shows a deviation in the value
of pV. Hydrogen liquefies at 21 K. Explain the possible deviations.
(6)
(17)
SECTION B: SOLUTIONS AND HINTS
Question 1
(Convert to correct units!)
1.1
pV = nRT
99,3 x 103  x 65 x 10-3  = n x 8,31 x (25 + 273) 
n = 2,61 mol N2 
n =
2,61  =
m
M
m

28
= 72, 98 g N2 
1.2
Nitrogen cannot be referred to as an ideal gas as it is a real gas . Ideal gases must
have a negligible mass and be small particles. At high pressures and low
temperatures the intermolecular forces cannot be ignored – this causes unexpected
low volumes and liquefaction. The volume cannot be ignored – under the same given
conditions the volume will not decrease as predicted for the ideal gas. Real gases only
obey the Gas Laws at higher temperatures and low pressures.
Page 2 of 6
GAUTENG DEPARTMENT OF EDUCATION SENIOR SECONDARY INTERVENTION PROGRAMME
PHYSICAL SCIENCES
Grade 11
SESSION 14
(LEARNER NOTES)
(Learn the conditions for ideal gas behaviour well! It is always asked)
1.3
1.4
The temperature is not low , the pressure is not high – under these conditions all
gases behave the same.
Helium, hydrogen 
Question 2
2.1
pV against T graph for the investigation of the relationship between p, V and T
Mark allocation
Heading, must show relationship 
pV (J)
Axes - label  and unit 
Correct line 
2.2
particles
avogadro
n =
=
0
T (K)
9,54  10 22

6,02  10 23
= 0,1585 mol H2 
(Give original equation first, manipulate to get the ratio)
pV = nRT 
pV
= nR
T
= 0,1585  x 8,31
= 1,32
2.3
At low temperatures  the attractive forces between the molecules become
significant so that the volume becomes smaller than predicted by the Ideal Gas
Law. pV will therefore also become smaller
(Learn this as a model answer for the pV deviation questions – it is often asked and the
only difference will be using another gas or comparing two different gases eg. H2 and
O2 in terms of amount of deviation)
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GAUTENG DEPARTMENT OF EDUCATION SENIOR SECONDARY INTERVENTION PROGRAMME
PHYSICAL SCIENCES
Grade 11
SESSION 14
(LEARNER NOTES)
SECTION C: ADDITIONAL CONTENT NOTES
AN IDEAL GAS
An ideal gas is a gas behaviour of which the Kinetic Molecular Theory is able to explain and
predict accurately if it is a gas which exhibits all the characteristics properties of gases. It
obeys the ideal gas equation:
pV = nRT
V – volume in m3 , T – temperature in K, p – pressure in Pa
Properties of an ideal gas
The particles are identical and in a state of constant random motion.
Molecules occupy no volume.
They exert no forces on one another other than when they collide.
The collisions between the particles are perfectly elastic.
Deviation from ideal gas behaviour
Gases that deviate from the ideal gas behaviour are called real gases.
Real gases deviate from ideal gas behaviour at high pressures and low temperatures.
Due to the fact that real gas particles occupy volume it therefore becomes increasingly
difficult to compress a gas as the pressure increased, until eventually a limit of
compressibility is reached.
o Real gases occupy volume.
o Forces exist between real gas particles and these forces cannot be ignored.
o The ideal gas equation is only valid for idea gases.
o pV for a real gas is greater than that of an ideal gas.
Graphs showing the deviation
p
p
V
V
1/V
pV
p
V
p
V
1/p
p
T
T
Page 4 of 6
GAUTENG DEPARTMENT OF EDUCATION SENIOR SECONDARY INTERVENTION PROGRAMME
PHYSICAL SCIENCES
Grade 11
SESSION 14
(LEARNER NOTES)
SECTION : D HOMEWORK
Question 1: 20 minutes
1.1
State the assumptions of the Kinetic Molecular Theory
1.2
State Boyle’s Law
a.
in words
(2)
b.
in the form of a mathematical equation
(2)
Show graphically the relationship between pressure and volume of an enclosed gas at
a constant temperature
(2)
1.3
1.4
(6)
What relationship exists between the pressure and the temperature of a gas of
constant volume?
1.5
What is temperature?
1.6
Give the complete form of the general gas equation and state what each symbol
(2)
(2)
represents and its units.
(5)
1.7.1 Under what conditions will the behaviour of a real gas correspond to that of an ideal
gas?
(2)
1.8
When do they deviate?
(2)
(25)
Page 5 of 6
GAUTENG DEPARTMENT OF EDUCATION SENIOR SECONDARY INTERVENTION PROGRAMME
PHYSICAL SCIENCES
Grade 11
SESSION 14
(LEARNER NOTES)
SECTION : E SOLUTIONS TO SESSION 13 HOMEWORK
SECTION A: TYPICAL EXAM QUESTIONS
Question 1
1.1
1.2
All matter consist of tiny randomly moving particles 
There are spaces between the particles of matter, largest in gases and smallest in
solids and because of that gases are compressible 
Because the particles are in constant random motion, they collide with one another and
with the walls of the container and gas pressure is a measure of these collisions per
unit area. 
An increase in temperature causes an increase in pressure due to more collisions
Collisions are elastic 
Temperature of the gas relates to the average kinetic energy of the particles 
a.
Boyle’s Law – the pressure of an enclosed gas is inversely proportional to the
volume, provided
the temperature remains constant. 
b.
Mathematically: p1V1 = p2V2
1.3
V

1.4
P
Directly proportional

1.5
A measure of the average kinetic energy of the particles. 
1.6
pV = nRT 
p – pressure in Pa 
V – volume – m3 
n – number of moles
T – temperature in K
1.7
At high temperature and low pressures. Real gases deviate at low temperatures and
high pressure. 
1.8
At high pressure and low temperature
The SSIP is supported by
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