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Moles


Moles


Measuring samples

How can we quantify the amount of substance in a sample?

You might be used to answering this

question using mass, giving your units

in grams.

What if we wanted to know

how many atoms there are?

A unit of measurement called

the mole is used instead.

There are too many to count…


What is a mole?

A mole is an SI unit that is used to denote the number of

atoms or molecules in a given amount of substance. The

symbol for the unit mole is mol.

This is equal to lots of atoms indeed. One mole of any

substance contains 602,000,000,000,000,000,000,000

(6.02 × 1023) particles.

A mole is defined as the number of atoms

in exactly 12g of carbon-12.


What is a mole? (2)

What is the mass of one mole of hydrogen atoms?

For example, the relative

atomic mass of carbon is 12,

so one mole of carbon

atoms weighs 12 grams.


Moles and mass

The mass of one mole of a substance is numerically

equal to its relative atomic mass (or relative formula

mass) in grams.

How is it possible to know the quantity of each substance that

contains the same number of particles as 12 g of carbon-12?

How many moles are there in the following:

24 g of magnesium?

24 g of carbon?

20 g of calcium?

1 mol

2 mol

0.5 mol


How many moles?


Avogadro’s number

One mole of a substance contains the same number of

particles as one mole of any other substance.

The number of particles in one mole of a substance is

called Avogadro’s number.

For example,

Avogadro’s number = 6.02 × 1023

1 mole of carbon atoms weigh 12g.

1 mole of calcium atoms weigh 40g.

Therefore, there are 6.02 × 1023 atoms in 12g of carbon.

Therefore, there are 6.02 × 1023 atoms in 40g of calcium.Ca40

20

calcium

C

12

6

carbon


Avogadro’s number in context

If you collected 6.02 × 1023

marbles and spread them

over the surface of the Earth,

they would form a layer of

marbles 50 miles thick!

Avogadro’s number is named after Amedeo Avogadro,

an Italian scientist working in the early 19th century.

6.02 × 1023 really is a

remarkably large

number of particles.

crust

mantle

marbles


Moles

Moles can be used to measure atoms, ions, electrons

or molecules.

How many moles are there in the following:

17 g of ammonia (NH3)?

12g of carbon?

44 g of carbon

dioxide (CO2)?

1 mol

1 mol

1 mol

28g of nitrogen (N2)? 1 mol

The number of particles in each of the above substances

will be equal to Avogadro’s number (6.02 × 1023).


What is the mass of one mole?


Moles


molar mass (g/mol)

Mole equation

The number of moles in a sample can be calculated

using this equation:

The molar mass is the mass in grams of one mole of a

substance. For elements, this is simply the RAM value in

grams/mol.

number of moles (mol) =mass of substance (g)

Some elements exist as molecules, for example, O2. The

molecular mass will need to be used for these elements

rather than the atomic mass.


11

28

molar mass

Mole equation: example calculations

1. How many moles of iron are in 28g of pure iron?

number of moles = = 0.5 moles of iron56

2. How many moles of CO2 are in 11g of carbon dioxide?

relative formula mass = 12 + (16 × 2) = 44

number of moles = = 0.25 moles of CO244

number of moles =mass of substance


Mole equation rearrangement

moles

mass

molar

mass

mass = moles × molar mass

The mole equation can also be rearranged. This can be

done by using an equation triangle (below).

Example

1. What is the mass of 3.5 mol

of pure iron, Fe?

mass = 196g of iron= 3.5 x 56


Mole calculations


Moles


Glossary


Multiple-choice quiz

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