Moles: worked examples

Magnesium

What is the mass of one mole of magnesium?

The real question is: what is the weight of 6 x 10²³ atoms of magnesium?

To answer it, you’ll need to find the weight of one atom of magnesium and multiply by 6 x 10²³.

 

What does one atom of magnesium weigh? How many protons and neutrons does it have? To work this out, look up the mass number of magnesium (24).

As there are 24 protons and neutrons in a magnesium atom, it will weigh: 24 x 1
6 x 10²³ g                                                                                                                                                                                                                                                                                                                                                

One mole of magnesium would weigh 6 x 10²³ x 24 x 1
6 x 10²³ g                                                                                    

6 x 10²³ x 24 x 1
6 x 10²³ = 24g                   

Answer: the mass of one mole of magnesium atoms is 24g.

 

Now we’ll find the mass of one mole of Carbon C, Silver Ag and Gold Au.

Remember, the real question is: what is the weight of 6 x 10²³ atoms of carbon, silver or gold?

So find the weight of one atom of carbon, silver or gold and multiply by 6 x 10^23.

 

Carbon

To find the weight of 1 mole of carbon atoms, you first need to work out how many protons and neutrons it has (12).

1 proton or neutron weighs 1
6 x 10²³ g                                   

12 protons and neutrons = 12 x 1
6 x 10²³ g                                                   

One mole of these = 6 x 10²³ x 12 x  1
6 x 10²³ = 12g

Answer: the weight of 1 mole of carbon = 12g

 

Silver

To find the weight of 1 mole of silver atoms, first work out how many protons and neutrons silver has (108).

 

1 proton or neutron weighs 1
6 x 10²³ g                                        

108 protons and neutrons = 108 x 1
6 x 10²³g

One mole of these = 6 x 10²³ x 108 x 1
6 x 10²³g = 108 g

 

Answer: the weight of 1 mole of silver is 108g

 

Gold

To find the weight of 1 mole of gold atoms, find out how many protons and neutrons a gold atom has (197).

 

1 proton or neutron weighs1
6 x 10²³ g

197 protons and neutrons = 197 x 1
6 x 10²³g

One mole of these = 6 x 10²³ x 197 x 1
6 x 10²³g = 197g       

Answer: the weight of 1 mole of gold is 197g 

 

Methane

What is the mass (weight) of 1 mole of CH₄ (methane) molecules?

The real question is: what is the weight of 6 x 10²³ atoms of one carbon atom and 4 hydrogen atoms?

How many protons and neutrons does carbon have: 12

How many protons and neutrons does 4 hydrogen atoms have: 4                   

12 + 4 = 16 protons and neutrons

 

1 proton or neutron weighs 1
6 x 10²³ g                                             

16 protons and neutrons weigh 16 x 1
6 x 10²³ g                                                      

One mole = 6 x 10²³ x 16 x 1
6 x 10²³ g = 16g

                                       

Easy!  Aren’t moles easy!

 

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