mole handout

7/27/2019 Mole Handout

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The mole

Chemistry



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



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



Calculating the number of moles

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

mass/ RFM= 13.2/132=0.1 moles



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.



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



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



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)




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




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



Useful Points to note

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

Iron will displace copper from copper sulphate .



Problem 1

Calculate the volume of hydrogen produced when

6.5g of Zn is reacted with excess dilute sulphuric

acid



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



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



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



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)



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

mole handout

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