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Unit 1 – Module 1

Lesson 3 - The Mole

Specification statements

• Explain the terms;– Amount of substance

– Mole (symbol ‘mol’) as the unit for amount of

substance

– Avogadro’s constant NA, as the number of

particles per mole (6.022 x 1023 mol-1)

• Define and use the term ‘molar mass’ (units gmol-1)

as the mass per mole of a substance.

Most

Describe the terms moles and molar mass

Some

Explain how to use moles and molar mass in

calculations

Starter

Look at the substances on show. 12g of Carbon

32 g of Sulphur

27 g of Aluminium

What do they have in common?

Answer

They all contain exactly the same number of

particles. We call this amount 1 Mole.

The Mole

• 1 mole is the amount of substance which

contains as many particles as there are atoms

in exactly 12 grams of 12C.

• This value is 6.022 x 1023 particles.

• For an element it is the Ar in grams

• For example 1 mole of hydrogen atoms has a

mass of exactly 1 gram.

Avogadro’s Constant

• Avogadro said that in 1 mole of any substance

there 6.022x1023 particles. This is called

Avogadro’s constant and is given the symbol

NA. It has the units mol-1.

• Even though there are the same number of

particles in a mole, they have different masses

according to their Ar.

Thinking big

• The Avogadro number is really really big! Lets

look at some other big numbers.

• There are approximately 400 billion stars in a

typical galaxy like the milky way.

• There are approximately 80 billion galaxies in

the universe

• There are approximately 3 x 1023 stars in the

observable universe.

• There are approximately 15 times as many

water molecules in a cup of tea than there are

Molar Mass

This is defined as the mass of 1 mole of a substance. It

has the units gmol-1.

When the number of moles equals 1 then this number

is also equal to;

• Relative atomic mass (Ar) or

• Relative molecular mass (Mr).

Water molecules have Mr = 18.

therefore 1 mole of water has a mass of 18 grams.

The molar mass of water = 18 gmol-1

1 mole of 12C contains exactly 12 grams of carbon atoms.

Which of the statement(s) below is/are correct?

1. 1 mole of oxygen has a mass of exactly 8 grams

2. 1 mole of sodium has a mass of exactly 23 grams

3. 1 mole of chlorine molecules have a mass of exactly 71 grams

4. 1 mole of water molecules have a mass of exactly 10 grams

How would you correct the incorrect statements?

Calculating number of moles

Number of moles (mol) = mass (g)

molar mass (gmol-1)

This equation can be rearranged to calculate mass or molar mass as well as number of moles.

Mass (g)

Mr (gmol-1)

No. of moles

• How many moles of Ca are there in 120g of Ca?

Ar Ca = 40, therefore Mr Ca = 40 gmol-1

number of moles = 120

40

= 3.0 mol

AfL – using whiteboards

Calculate the amount in moles in the following;

1. 32.1 grams of sulfur atoms

2. 50.0 grams of CaCO3

3. 6.35 grams of copper atoms

4. 400 grams of NaOH

5. 69g of Pb

6. 5.30g of Na2CO3

Calculating masses

Worked example

What mass of NaCl contains 10 moles of NaCl particles?

Mr NaCl = 23 + 35.5 = 58.5

therefore Mr of NaCl = 58.5 gmol-1

From the equation on the previous slide ;

Mass = moles x Mr

Mass = 10 mol x 58.5 gmol-1 = 585 grams

AfL – using whiteboards

Calculate the mass of

1. 0.013 moles of Cl2

2. 2.00 moles of SO3

3. 0.25 moles S8

4. 0.15 moles of MgSO4.7H2O

Plenary

• Write your own definitions of ;– The mole

– Molar mass of an element

– Molar mass of a compound

• Compare to statements from mark schemes

• Learn these statements!

Homework

• Text book page 11, questions 1 and 2

Describe the terms moles and molar mass

Explain how to use moles and molar mass in

calculations