The Mole - Wizznotes.com
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Chemistry
The mole
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 contain 6x1023 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 a balanced
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)
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
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-Zinc
Fe- 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 moles should
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 moles should
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