10.03.09 Moles of Gas (HT)

Syllabus

  • Equal amounts in moles of gases occupy the same volume under the same conditions of temperature and pressure.

  • The volume of one mole of any gas at room temperature and pressure (20oC and 1 atmosphere pressure) is 24 dm3 .

  • The volumes of gaseous reactants and products can be calculated from the balanced equation for the reaction.

  • Students should be able to:

  1. calculate the volume of a gas at room temperature and pressure from its mass and relative formula mass

  2. calculate volumes of gaseous reactants and products from a balanced equation and a given volume of a gaseous reactant or product.

What does this mean?

What is the Molar Gas Constant?

Gas particles are very small compared to the space between them.

So gases with slightly larger molecules don't actually occupy any additional space than gases with smaller molecules.

And this means that 1 mole of any gas always occupies the same space.

This is called the Molar Gas Volume.

And at Room Temperature and Pressure this volume is 24,000 cm3 or 24 dm3 (24 Litres)

Obviously particles occupy some space. Just not enough to be worth considering.

Finding the volume of a known number of moles.

The Molar Gas Constant means that if we know how many moles of gas there are, we know how much space it will occupy and we don't have to worry about which gas it is.

So, how much space would 3 moles of Oxygen occupy?

1 mole occupies 24,000 cm3 so 3 moles will occupy 3 x 24,000 cm3 = 72,000 cm3

So, how much space would 3 moles of Steam occupy?

1 mole occupies 24,000 cm3 so 3 moles will occupy 3 x 24,000 cm3 = 72,000 cm3

Finding the number of moles in a known volume of gas.

How many moles are there in 48,000 cm3 of Methane?

1 mole occupies 24,000 cm3

So the number of moles is 48,000 cm3 ÷ 24,000 cm3 = 2 moles

How many moles are there in 4000 cm3 of Nitrogen?

1 mole occupies 24,000 cm3

So the number of moles is 4000 cm3 ÷ 24,000 cm3 = 0.16 moles

How many moles are there in 18 dm3 of Fluorine?

1 mole occupies 24 dm3

So the number of moles is 18 dm3 ÷ 24 dm3 = 0.75 moles

How many moles are there in 0.325 cm3 of Helium?

1 mole occupies 24,000 cm3

So No. moles is 0.325 cm3 ÷ 24,000 cm3 = 1.35 x 10 -5 moles

Using in an equation

EXAMPLE 1

If 24 dm3 of Hydrogen reacts completely with Oxygen, what volume of Oxygen is used and what volume of Water vapour is made?

2H2(g) + O2(g) --> 2H2O(g)

24 dm3 of Hydrogen is 1 mole - that will make 2 mole of water vapour.

Which is also 24 dm3.

And 1 mole of Hydrogen only reacts with 0.5 moles of Oxygen.

Which is 12dm3.

So we can simply use the mole ratio to multiply the volume of other gases

Example 2

If 258 cm3 of Oxygen reacts what are the volumes used/made of other gases in the following reaction?

O2:H2O

1:1

So, also 258 cm3 of water vapour

O2:CO2

2:1

So, 258 cm3 /2 of Carbon Dioxide = 129 cm3

O2:CH4

2:1

So, 258 cm3 /2 of Methane = 129 cm3

Example 3

If 258 cm3 of Propane reacts what are the volumes used/made of other gases in the following reaction?

C3H8:H2O

1:4

So, 4 x 258 cm3 of water vapour = 1032 cm3

C3H8:CO2

1:3

So, 258 cm3 x 3 of Carbon Dioxide = 774 cm3

C3H8:O2

1:5

So, 258 cm3 x 5 of Oxygen= 1290 cm3

Video

Past Paper Questions

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