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 The mole

Chemistry

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The mole

• Very important concept

• Used in almost all calculation in chemistry

• What is a mole?

•  A mole is the amount of substance that contains

6x1023 elementary units (ions, molecules and

atoms).

•

1 mole of sodium chloride ions would contain6x1023 ions

• 1 mole of carbon dioxide would contain 6x1023

molecules

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The mole

Mole = given mass/ R.F.M

R.F.M- Relative Formula Mass

R.F.M= Sum of the Relative Atomic Mass (R.A.M)

R.F.M of NaCl= 23+35.5= 58.5

R.F.M of ammonium sulphate- (NH4)2SO4 =

2x(14+4) + 32+ 4(16)= 132

 

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Calculating the number of moles

Number of moles in 13.2g (NH4)2SO4 = given

mass/ RFM= 13.2/132=0.1 moles 

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The mole and the equation

Calculate how many grams of Calcium Chloride

will be formed when 2 grams of calcium is mixed

with an excess of chlorine gas.

To solve this problem, we must first write abalanced equation of the reaction taking place.

Then determine the mole ratio.

This is shown on the next slide.

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Solution

 

Ca + Cl2 −> CaCl2  

There is a 1:1 ratio of Calcium and Calcium

Chloride

This means that there will be a 1:1 mole ratio of 

calcium reacted to calcium chloride produced

 

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Solution continued

No. moles calcium= given mass/ molar mass

= 2g/40= 0.05 moles

Since there is a 1:1 mole ration

0.05 moles of Calcium Chloride will be formed

Since moles = mass/R.F.M

mass= R.F.M x moles= (40+ 2(35.5) x 0.05

=111 x 0.05 = 5.55g CaCl2

 Answer= 5.55g CaCl2

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Useful points to note

1 mole of gas, occupies 22.4 dm3 (This will be

important when calculating volumes of gases

produced).

Reactivity points to note Reactive metals (e.g. Group 1,2 and 3 metals and

metals such as Zn, Fe and Cu) react with dilute

acids to form a salt and hydrogen gas

e.g.

Zn(s) + H2SO4(aq)−> ZnSO4(aq) + H2(g)

 

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Useful points to note

More reactive metals will displace less reactive

metals from solution according to order of 

reactivity.

The order of reactivity is shown below for some of the common metals you will come across during

your CSEC studies.

K>Na>Ca>Mg>Al>Zn>Fe>Cu>Ag

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Useful points to note

You should know most of these symbols however 

you may not be familiar with Zn, Fe, Cu and Ag.

These are defined below:

Zn-ZincFe- Iron

Cu- Copper 

 Ag-Silver 

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Useful Points to note

Fe(s) + CuSO4 (aq )−> FeSO4(aq) + Cu(s)

Iron will displace copper from copper sulphate .

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Problem 1

Calculate the volume of hydrogen produced when

6.5g of Zn is reacted with excess dilute sulphuric

acid

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Solution

Equation

Zn(s) + H2SO4(aq)−> ZnSO4(aq) + H2(g)

No moles of Zn reacted = given mass/R.F.M =6.5/

65 No. moles of Zn reacted = 0.1 Moles

What is the mole ratio of Zn reacted to H2 

liberated

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Solution

1:1 ratio

From 1:1 mole ration, the no. moles of hydrogen

gas is liberated = 0.1moles

Since 1 mole occupies 22.4 dm3 , then 0.1 molesshould occupy 2.24 dm3( 0.1 x 22.4). Answer =

2.24 dm3

 

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Solution

1:1 ration

From 1:1 mole ration, the no. moles of hydrogen

gas is liberated = 0.1moles

Since 1 mole occupies 22.4 dm3 , then 0.1 molesshould occupy 2.24 dm3( 0.1 x 22.4). Answer =

2.24 dm3

 

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Problem 2

Calculate the volume of oxygen gas formed

during the electrolysis of 18 moles of water.

Equation is shown below:

2H20(l) −> O2(g) + 2H2(g)

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Solution

Mole ratio

2 mole of water: 1 moles of oxygen gas

Hence 18 moles of water would produce 9 moles

of oxygen gas. We know that 1 mole of gas occupies 22.4 dm3

Then 9 moles of oxygen will occupy 9 x 22.4 =

201.6 dm3