1. Lesson 12
Ideal Gases

2. Refresh
What volume of sulfur trioxide, in cm3, can be prepared using 40 cm3 sulfur dioxide and 20 cm3 oxygen gas by the following reaction? Assume all volumes are measured at the same temperature and pressure.
2SO2(g) + O2(g) → 2SO3(g) 20 40 60 80

3. We Are Here

4. Lesson 12: Ideal Gas Equation
Objectives:
Understand the ideal gas equation
Complete a circus of short experiments to explore the ideal gas equation
Perform calculations using the ideal gas equation

5. Molar Volume of a Perfect Gas
We learnt about the molar volume of gases last lesson….how can they be the same?
The distance between particles is much bigger than the size of the particles….so particle size makes very little difference:
The blue particle is twice the size of the red particle, but the blue particles are not taking up twice the amount of space.
In reality, the relative distance between the molecules is much much greater than this.
10 units

6. The Ideal Gas Equation The volume a gas takes up is determined by:
Pressure
Temperature
Moles of gas
This combines to form the ideal gas equation
PV = nRT
Where:
P = pressure in Pa
V = volume in m3
n = moles of gas
R = gas constant, 8.31 J K-1 mol-1, this appears in many places in chem
T = temperature in K

7. Ideal Gas Assumptions Particles occupy no volume
Particles have zero intermolecular forces
These are not always valid, particularly at:
Low temperature – when intermolecular forces become significant
High pressure – when particle volume becomes significant

8. Study the equation and predict the lines you would expect on these graphs (assuming the third factor is fixed):
PV = nRT
Temperature
Volume
Temperature
Pressure
Volume
Pressure

9. Example 1: g of unknown gas A, occupies 846 cm3 at 500K and standard pressure. What is it’s molecular mass?
State ideal gas equation
Rearrange for chosen subject
Sub in numbers with unit conversion and evaluate
𝑀 𝑚 𝐴 = 𝑚𝑎𝑠𝑠 𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 = =60.0
Complete further calculations

10. Example 2 (your turn): A fire extinguisher containing 45
Example 2 (your turn): A fire extinguisher containing 45.4 mol CO2 has a volume of 3000 cm3. What is the pressure inside at 50OC?
State ideal gas equation
Rearrange for chosen subject
Sub in numbers with unit conversion and evaluate

11. Where doe this come from?
One last example:
The volume of an ideal gas at 54.0 °C is increased from dm3 to 6.00 dm3. At what temperature, in °C, will the gas have the original pressure?
Use a modified version of the ideal gas equation:
Since original and final pressure should be the same, we can remove this from the equation as they cancel out:
3.00 / = 6.00 / T2
T2 = (6.00 x / 3.00) = 653 K
Where doe this come from?
54.0 converted to Kelvin by adding 273

12. Key Points The Ideal Gas equation: PV = nRT Also: Provided that:
Molecules have zero volume
Molecules experience no attraction to each other