The Mole
Chemistry: AQA A Level, topic 1.2.2
Moles
The amount of a substance is measured using a unit called a mole (mol).
What is a mole?
- A mole is simply a massive number of particles.
- You can have a mole of anything: laptops, sunglasses and, of course, atoms (this is what we care about!).
- A mole of things contains 6.02 ×1023 things.
Carbon-12
- The value of the mole is set so that a mole of carbon-12 atoms weighs 12 grams.
- This is important because the mass of one carbon-12 atom is 12 amu.
- So the mole converts between amu and grams:
- 1 g = 1 mol × 1 amu
Convenience
- Atoms are really tiny and so it's very difficult to measure a single atom.
- By using the mole, we can do calculations about the amount of stuff we have in any reaction.
Avogadro's constant
- The value of Avogadro's constant is 6.02 ×1023.
- So one mole contains Avogadro's constant of things.
- Number of particles = number of moles × Avogadro's constant
Concentration
A mole isn't very useful when dealing with liquids or solutions. It's easier to deal with the volume of fluid.
Solutions
- It's much easier to look at a liquid and measure its volume than to evaporate any water and measure the mass of the remaining solid.
- So we need a value that will let us know how much of a solid is dissolved in a solution simply from its volume.
- If we know the concentration of a solution, we can calculate the number of moles in any sample volume of the solution.
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Concentration
- Concentration is defined as moles per unit volume. The usual units of concentration are moles per litre.
- Litre is often written as dm3.
- So moles per litre is mol ÷ dm3.
- This is often written as moldm-3.
An example
- Three moles of NaCl are dissolved in half a litre of water.
- Concentration = number of moles ÷ volume
- Concentration = 3 mol ÷ 0.5 dm3
- Concentration = 6 moldm-3
Mole Calculations
Earlier we saw that a mole of a molecule with an Mr of 200 weighs 200 g. This is a very useful property.
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Mole calculations
- We can calculate the number of moles present in a sample if we know its mass, and its Mr:
- Moles = mass ÷ Mr
A worked example
- Benzene has an Mr of 78. How many moles of benzene are in 7.8 g of pure benzene?
- Moles = mass ÷ Mr
- Moles = 7.8 g ÷ 78
- Moles = 0.1 mol
Test yourself on The Mole
What is a mole?
Which of the following is true of moles?
What is the value of Avogadro's constant?
What are the units of concentration?
Which of the following is true of concentration?