1.2.2

# The Mole

Test yourself

## 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.

• 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.

### 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.

### 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