Download Relative formula mass

Chemicals of the Natural
Environment.
Lesson 9
Learning objective: To calculate how
much metal can be extracted from its
mineral.
• Must: Be able to calculate the relative formula
mass for any compound.
• Should: Be able to calculate the mass of a metal
that can be extracted form its mineral.
• Should: Be able to write balanced symbol
equations.
• Keywords: Atomic mass, relative formula mass,
percentage composition, reactants and products.
• Starter: What do these numbers mean
CONNECT
 What
is the total number of atoms in the
following formulae?
NaOH
H2O
CaCO3
H2SO4
Ca(OH)2
 A small
number just talks about the element it
is behind. E.g. O2 = 2 x O
 Small numbers outside the bracket mean
multiply everything inside the bracket by
that number E.g. (CO3)3 = 3 x C, 9 x O
Relative atomic mass
Relative atomic masses
1. Atoms of different elements have
different masses
2. The relative atomic mass (Ar) is the
mass of the elements compared to the
mass of a hydrogen atom
Relative Formula Mass
1. The relative formula mass (Mr) is the
relative mass of a molecule compared
to the mass of a hydrogen atom
Calculating relative formula mass.
• Top Tips.
• 1. The relative atomic mass is usually at the top –
it is always the highest number.
• 2. When calculating the relative formula mass,
• a) Write out all the different atoms (Capital
letter).
• b) Write down the number of each atom.
• c) Multiply the number of each type of atom by
its atomic mass.
• d) Add all the masses together.
An example.
•
•
•
•
•
•
•
Calculate the RFM – for calcium carbonate.
Formula CaCO3
Type, number and atomic mass of each atom;
Ca x 1 x 40 = 40
C x 1 x 12 = 12
O x 3 x 16 = 48
Total = 100 – Relative formula mass of CaCO3.
Relative formula mass
Relative formula mass, Mr
12/05/2017
The relative formula mass of a compound is blatantly the relative
atomic masses of all the elements in the compound added together.
E.g. water H2O:
Relative atomic mass of O = 16
Relative atomic mass of H = 1
Therefore Mr for water = 16 + (2x1) = 18
Work out Mr for the following compounds:
1) HCl
H=1, Cl=35 so Mr = 36
2) NaOH
Na=23, O=16, H=1 so Mr = 40
3) MgCl2
Mg=24, Cl=35 so Mr = 24+(2x35) = 94
4) H2SO4
H=1, S=32, O=16 so Mr = (2x1)+32+(4x16) = 98
5) K2CO3
K=39, C=12, O=16 so Mr = (2x39)+12+(3x16) = 138
More examples
CaCO3
40 + 12 + 3x16
HNO3
1 + 14 + 3x16
2MgO
2 x (24 + 16)
3H2O
3 x ((2x1) + 16)
4NH3
2KMnO4
3C2H5OH
4Ca(OH)2
12/05/2017
100
80
PROGRESS
CHECK
 Mr
of CO2
ADD
 Mr
of NaCl
ADD
 Mr
of MgO
142.5
PROGRESS
CHECK
Mr
of CuSO4
SUBTRACT
Mr
of Al2O3
58
PROGRESS
CHECK
Mr
of H2O
35
ADD
Mr
of ammonia
Calculating percentage mass
12/05/2017
If you can work out Mr then this bit is easy…
Percentage mass (%) =
Mass of element Ar
Relative formula mass Mr
x100%
Calculate the percentage mass of magnesium in magnesium oxide, MgO:
Ar for magnesium = 24
Ar for oxygen = 16
Mr for magnesium oxide = 24 + 16 = 40
Therefore percentage mass = 24/40 x 100% = 60%
Calculate the percentage mass of the following:
1) Hydrogen in hydrochloric acid, HCl
2) Potassium in potassium chloride, KCl
3) Calcium in calcium chloride, CaCl2
4) Oxygen in water, H2O
Calculating the mass of a metal that
can be extracted from its mineral.
• How much Fe could you get from 100kg of
Fe2O3?
• There are 2 steps;
• 1. Calculate the percentage mass
(composition) for the element in this case Fe.
• 2. Multiply the percentage mass (from step 1)
by the mass of the mineral
An example.
•
•
•
•
Percentage composition of Fe in Fe2O3
Mass of Fe in Fe2O3 = 2 x 56 =112
Mass of Fe2O3 = 2 x 56 + 3 x 16 = 160
Percentage Fe in Fe2O3 = 112 x 100 = 70%
160
• Mass of Fe in 100 kg of Fe2O3 = 70 x 100
•
= 70 kg
• Click here for more examples of percentage
composition problems.
DEMONSTRATE
 How

much gold is in your mobile phone?
Use the procedure to calculate the percentage
composition of gold in a mobile phone
More problems.
• 1. What mass of Al could be made from 1 tonne
of Al2O3?
• 2. What mass of Na could be made from 2 tonnes
of NaCl?
• 3. The main ore of chromium is FeCr2O4. What is
the percentage mass of Cr in FeCr2O4?
• 4. 1000 tonnes of copper ore are dug out of the
ground. Only 1% of this is the pure mineral,
CuFeS2. What mass of the Cu could be made from
1000 tonnes of the ore?
Conservation of Mass
Show that mass is conserved in this reaction.
ZnCO3
ZnO + CO2
You need to use the ideas of Mr
Simple chemical reactions
H
Mg
Cl
O
H
Magnesium +
oxide
MgO
Cl
Cl
Mg
H
Cl
Magnesium
chloride
+
Water
2HCl
MgCl2
+
H2O
+
Cu
O
O
S
O
Cu
Mg
Mg
+
Copper
sulphate
CuSO4
O
S
O
O
Magnesium +
H
Hydrochloric
acid
O
Mg
O
Copper
+
Magnesium
sulphate
Cu
+
MgSO4
O
Balancing reaction equations
This sequence of slides includes two approaches to
balancing reaction equations. Individual slides may
be copied elsewhere or used as templates.
1 Using symbols and no text
click here
2 Using text prompts to explain
each step of the process
click here
To return to this page,
click the button to the top right of any slide.
Original slide
prepared for the
2Mg +
O2  2 MgO

Symbols Mg + O2
Original slide
prepared for the
2 Al
+ 1½
I23
23
AlI

Symbols Al +
I2
Original slide
prepared for the
Write a balanced equation
sodium + water  sodium hydroxide + hydrogen
formula
each
substance
Checkthe
thein
equation
balanced:
Add Write
numbers
front of
ofis
the
ringed
formula
asofitreactant
is ringedof
above
countsothe
number
and
productsymbols
elements
that
the total
number
reactant
= total
number
of product
symbols
sodium
hydroxide
is ionic:
Reactants:
2Na 4H
2O
what are the formulae of its ions?
Products:+ 2Na 4H- 2O
Na and OH
It’s balanced!
2 Na + 2 H2O  2 NaOH + H2
Original slide
prepared for the
References
First example, with symbols only:
Frank Harriss of Malvern College
Second example, with text prompts: Steve Lewis
Original slide
prepared for the
Some examples
2Mg
O2
2 MgO
Zn
+ 2 HCl
ZnCl2
2 Fe
+ 3Cl2
2 FeCl3
NaOH
CH4
Ca
+
+
HCl
+ 2 O2
NaCl
CO2
+
+
H2
H 2O
+ 2H2O
+ 2 H2O
Ca(OH)2
+
+
H2SO4
Na2SO4
+ 2H2O
2 CH3OH
+ 3 O2
2 NaOH
2 CO2
+ 4H2O
H2
Balancing equations