Transcript Volumetric analysis

4.3 Volumetric analysis
Learning outcomes
To understand the apparatus used in
volumetric analysis and correct titrimetric
procedure.
Students should know
the following key words:
Volumetric analysis,titration,standard and
non standard solutions
Volumetric analysis
A titration is a lab procedure where a
measured volume of one solution (burette) is
added to a known volume (flask) of another
solution until the reaction is complete
Standard solutions
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A standard is a solution of precisely known
concentration
It must be available in a highly pure state
It must be stable in air
It must dissolve easily in water
It should have a fairly high relative molecular wt
It should under go a complete and rapid
reaction
Non standard solutions
Sodium hydroxide absorbs carbon dioxide
from atmosphere
 HCl can produce chlorine gas in reactions
and liberate hydrogen when exposed to
air
 Nitric acid can act as an oxidising agent
interfering with reactions
 Sulphuric acid absorbs water form the air
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Sodium Carbonate and Hydrochloric acid
Na2CO3
1 mole
+ 2HCl  2NaCl +
2 moles
2 moles
H 2O +
1 mole
CO2
1 mole
EQUIPMENT
Apparatus used
Burette
 Volumetric flask
 Beaker
 Pipette
 Funnel
 Indicator
 White tile
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Burette titration procedures
Precautions when using equipment
Burette must be vertical, use and then
remove funnel, check meniscus, rinse with
de-ionised water and then given solution.
 In using a Pipette rinse with de-ionised
water first and then with given solution.
Check meniscus. Do not blow out
remainder of liquid into flask and keep tip
of pipette in contact with flask
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Precautions
Conical flask should not be rinsed with
solution it is to contain and swirl
 In using a Volumetric flask the last few
cm³ must be added so that the meniscus
rest on calibration mark
 Invert stoppered flask to ensure solution is
homogeneous/uniform
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Why is a conical flask, rather than a
beaker, used in the experiment?
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To allow easy mixing of the contents, by
swirling.
Why is the funnel removed from
the burette after adding the
acid solution?
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So that drops of solution from the funnel
will not fall into the burette.
In using a burette, why is it
important
(a) to rinse it with a little
of the solution it is going
to contain.
(b) to clamp it vertically.
(c) to have the part below
the tap full?
Solution (a) Rinsing
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To remove any residual water, and so
avoid dilution of the acid solution when it
is poured into the burette.
Solution (b) clamp vertically
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To enable the liquid level to be read
correctly
Solution (c) Full tap
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To ensure that the actual volume of
liquid delivered into the conical flask is
read accurately.
The following procedures were
carried out during the titration:
The sides of the conical flask were
washed down with deionised
water.
The conical flask was frequently
swirled or shaken.
Give one reason for carrying out
each of these procedures.
To ensure that all of the acid
added from the burette can
react with the base.
To ensure complete mixing of
the reactants
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Why is a rough titration carried out?
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To find the approximate end-point. This
information enables the subsequent
titrations to be carried out more quickly.
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Why is more than one accurate titration
carried out?
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To minimise error by getting accurate
readings within 0.1 cm³ of each other.
Calculation example
Volume of acid Va
 Volume of base is Vb
(cm3) is the titration
(cm³ ). Usually placed
figure from burette
in the conical flask.
 The concentration of
 Cb is the
acid is Ca (mol)
concentration of the
base
 na is the mol of full
balanced equation per  na is the mol of full
litre
balanced per litre
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Calculations
USE FORMULA Va. Ca = Vb. Cb
na
nb
 Va = 37 cm³
 Ca is unknown
 na = 2
 Vb = 25 cm³
 Cb = 0.1 mol
 nb = 1
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Va. Ca = Vb. Cb
na
nb
37 cm³ × Ca =
25 cm³ × 0.1mol
2
1
Ca = 25 × 2 × 0.1
37
= 0.13 mol/L
EXAM QUESTIONS
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Look out for dilution factors e.g vinegar
Choice of indicator
Type of vol flask given 1L OR 250 cm³ as you
have to adjust in your calculations
Take titre reading from burette and given vol of
solution is taken from conical flask
USE FORMULA Va. Ca = Vb. Cb
na
nb