Answering Questions on Redox Titrations I
Number 57
1
C
hem
F
actsheet
www.curriculumpress.co.uk
To succeed with this topic you need to:
•
Know and understand how to do
AS titration calculations ( Factsheets
No. 7 (‘Moles and Volumetric Analysis’) and No. 23 (‘How to Answer
Questions on Titration Calculations’))
Understand the whole concept of Redox Equilibria (Factsheet No.37,
No.45, and No.50).
•
Understand the basics of redox titrations (Factsheet No.51)
After working through this Factsheet you will have
•
met more examples of redox titrations
•
seen the ‘method’ being used on a range of examples
You may be given a problem that looks very different from ones you
have seen before. The way to deal with it is to remember the following
method:
(1) Write the
EQUATION
for the titration reaction (you may need to
combine half equations using numbers of e
−
to write the full
equation).
(2) Write down the
REACTING RATIO
from the balanced equation.
(3) Identify which of the reactants you can find the
NUMBER OF
MOLES
of using
moles
=
volume (cm
3
)
×
M(mol dm
3
)
1000
(4) Combine answers from (2) + (3) to find the
NUMBER OF MOLES
OF THE OTHER REACTANT.
(5) Look at the question –
WHAT DO YOU NEED TO FIND?
This will dictate which equation to use next i.e.
moles
=
volume (cm
3
)
×
M(mol dm
3
)
1000
or moles
=
grams
A
r
/
M
r
or % purity
=
mass of pure
×
100
mass of impure
Summary

you could remember the 5 stages as
EQ
→
RR
→
Moles
→
‘other moles’
→
Q
→
A
(1)
(2)
(3)
(4)
(5)
(6)
In the following examples the questions are underlined at key places so the
method can be explained e.g.
U2
= the second underlined section. Underlining
key points is often helpful in examination questions – and not just for the
topic we are dealing with now  as it helps you focus your attention on the
key facts and figures.
Before you start – remember this topic requires problem solving  so the
method is a way of thinking, not just a recipe sheet. It's worth taking your
time to make sure you really are happy with what is going on.
Example 1
Ammonium iron(II) sulphate crystals have the formula:
(NH
4
)
2
Fe(SO
4
)
2
.
x
H
2
O.
In an experiment to find the value of
x
,
4.25 g of crystals were dissolved
in water and dilute sulphuric acid and made up to
250 cm
3
.
A
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 Spring '11
 hard
 Equations, Mole, Redox, Sodium, Potassium iodide, Berlin UBahn, Chem Factsheet

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