ADVANCED LEVEL CHEMISTRY
PRACTICAL ASSESSMENT
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Candidate Number |
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Centre Number |
71637 |
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Name |
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Date |
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PARTITION COEFFICIENT
SKILL AREAS ASSESSED : 2 Manipulation
3 Measuring
& Recording
5 Concluding & Communicating
The purpose of the experiment is to determine the
partition coefficient of succinic acid between water and 2-methylpropan-l-ol
(called X for convenience).
INTRODUCTION
If
a solid is soluble in each of two liquids, which are themselves immiscible,
then it will divide itself between the two liquids in a fixed proportion. The ratio of the concentration of the solid
in the two liquids is called the partition coefficient (or distribution
constant).
APPARATUS CHEMICALS
250
cm3 reagent bottles succinic
acid solution
measuring
cylinders 2-methylpropan-l-ol (X)
10 cm3 pipettes 0.125M NaOH solution
50 cm3 burette phenolphthalein
indicator
PROCEDURE
1.
Place (20,30,40,50) cm3 of the aqueous
succinic acid solution provided into a measuring cylinder and add distilled
water, if necessary, to make the volume up to 50 cm3. Transfer to solution into a reagent bottle.
2. Using a
different measuring cylinder, add 50cm3
of X to the bottle.
3. Stopper
and shake well for about 3 minutes, then
allow to stand for 5 minutes.
4. Using a pipette filler withdraw 10.0 cm3
of the upper layer and transfer to a conical flask.
5. Add an
approximately equal volume of water to the flask and titrate with the standard sodium hydroxide using phenolphthalein
indicator.
6. Repeat steps 4 and 5.
7. By pipette, remove 10.0 cm3
of the lower layer and transfer to a conical
flask, titrating as before (steps 4-6).
[To
assist with sampling of the lower layer it is recommended that you put your
finger over the top of the pipette before inserting into the reagent
bottle. Try not to disturb the layers
as you insert the pipette into the lower layer. Once the pipette is in the lower layer put the pipette filler in
position.]
Present your
results in a suitable table.
1.
Which
of the two layers in the reagent bottle is the aqueous layer?
2.
What
is your average titre for the water layer?
3.
What
is your average titre for layer X?
4.
From
the results you have obtained calculate a value for the partition coefficient
of succinic acid between water and X.
5.
Collect
a set of titration results from the
rest of the class and use them to plot a graph of the titration readings. Use the graph to determine the partition
coefficient.
6.
What
sources of error are likely to be present in the experiment and how might they
be minimised?
ADVANCED LEVEL CHEMISTRY PRACTICAL ASSESSMENT
CHECKLIST
|
Candidate Number |
|
Centre Number |
71637 |
|
Name |
|
Date |
|
SKILL AREA 2 manipulation Student
Mark
Selects
appropriate measuring cylinders (2)
Use
of pipette (including filler) (2)
Thorough
mixing and time allowed for equilibrium
(2)
Apparatus
washed with correct solutions (2)
Addition
of indicator (1)
Use
of burette (including control of end point)
(3)
Correct
end point (2)
Correct
sampling technique (2)
Works
in a safe and efficient manner (2)
|
TOTAL |
0-1 |
2-5 |
6-8 |
9-12 |
13-16 |
17-18 |
|
SCALED MARK |
0 |
1 |
2 |
3 |
4 |
5 |
SKILL AREA 3 MEASURING & RECORDING Student
Mark
Correct
volumes by measuring cylinders (2)
Correct
volumes by pipette (2)
Burette
readings (2)
Construction
of a suitable table (2)
Appropriate
degree of precision (2)
Units
included (1)
Immediate
permanent recording of results (1)
|
TOTAL |
0-1 |
2-3 |
4-5 |
6-8 |
9-10 |
11-12 |
|
SCALED MARK |
0 |
1 |
2 |
3 |
4 |
5 |
SKILL AREA 5 CONCLUDING &
COMMUNICATING Student
Mark
Identification
of aqueous layer (1)
Averaging
of titres (2)
Calculation
of partition coefficient (2)
Drawing of
graph (3)
Partition coefficient from graph (2)
Consideration of errors
(2)
|
TOTAL |
0-1 |
2-3 |
4-5 |
6-8 |
9-10 |
11-12 |
|
SCALED MARK |
0 |
1 |
2 |
3 |
4 |
5 |