In this video, we're going to learn how to calculate gas volumes for any reaction.
In my previous video (molar volumes of gases), you found out that one mole of any gas occupies
24 litres per mol.
This is the same for all gases at room temperature and atmospheric pressure.
So, if we know that one mole of any gas occupies 24 litres (think 24, 1L soft drinks bottles),
then how many soft drinks bottles do you think are needed to contain two moles of any gas?
Pause and choose from one of the following: A)24 soft drinks bottles
B)
48 soft drinks bottles C) 12 soft drinks bottles
The correct answer was B, 48 soft drinks bottles.
This is because we doubled the number of moles f gas from one to two, and so this doubled
the volume of gas produced.
However, when chemical reactions take place, they don't always produce whole numbers of
moles of gas, and so you need a different method to calculate the gas volume.
This is where you can use a formula and the ratio given from a chemical reaction:
Gas volume produced (litres) = number of moles of gas x 24 litres per mole
So, let's pose a question.
In my previous video, we talked about environmental chemists wanting to know how much carbon dioxide
is produced from burning a fuel.
A major part of petrol is a hydrocarbon called hexane.
When it is combusted in a car engine, it produces carbon dioxide (a gas) and water.
We're going to try and calculate the volume of carbon dioxide that is produced from this
reaction?
C6H12 + 9O2 6CO2 + 6H2O The equation tells us that for 1 molecule
of hexane (1 mole), that 6 molecules (or 6 moles) of carbon dioxide are are produced.
This is in a ratio of 1 to 6.
So, what volume of carbon dioxide do you think is produced if one mole of hexane reacts with
oxygen?
Pause, use the formula and choose from on of the following volumes.
Continue, when you are ready.
24 litres 12 litres
144 litres.
The answer is C, 144 litres.
This is because 1 mol of hexane produces 6 mols of carbon dioxide.
So if one mole of a gas occupies 24 litres then 6 moles of a gas occupy 24 litres x 6
mols, which is equal to 144 litres.
However, what if you combust any amount of hexane?
The amount of hexane used, in mass and moles, will vary according to how long the car is
used.
Let's say that you use the car for 20 minutes and it combusts 20.5 moles of hexane how many
moles of carbon dioxide do you think will be produced?
Pause, select and continue when you're ready:
20.5 moles x 6 = 123 moles of CO2 20.5 moles  6 = 3.42 moles of CO2
Neither, only one mole of CO2 is produced.
The corect answer is A) because 20.5 moles x 6 = 123 moles.
Knowing this, you can use the gas volume formula to work out the volume of CO2 gas produced
by combusting 20.5 moles of hexane.
Pause, use the formula and then continue.
The answer is that 123 moles of CO2 x 24 litres per mol = 2952 litres of CO2.Did you get it
right?
In summary, the molar volume of any gas is always 24 litres per mol at room temperature
and pressure.
Using the example provided, you can use the ratio from any chemical eqaution to tell you
the number of moles of gas produced from the reactants.
This means you can always calculate the gas volume using the formula: number of moles
of gas x 24 litres per mol (at room temp and pressure).