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

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