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AVAILABLE
MARKS
CHEMISTRY
Assessment Unit AS 3
assessing
Module 3: Practical Examination 2
Mark Scheme
Annotation
1. Please do all marking in red ink.
2. All scripts are checked for mathematical errors. Please adopt the system of one
tick (
) equals [1] mark e.g. if you have awarded 4 marks for part of a question
then 4 ticks (
) should be on this candidate’s answer.
3. As candidates have access to scripts please do not write any inappropriate
comments on their scripts.
Section A
1 (a)
Rinse the pipette with distilled water and then drain cleaner [1]
Pipette 25cm
3
of the oven cleaner into the 500cm
3
volumetric flask and
make up to mark using distilled water [1]
(Stopper and) shake the flask/invert [1]
Using a pipette filler [1]
Rinse the pipette with the diluted solution and transfer 25cm
3
conical
flask [1]
(1 mark each to a maximum of four)
[4]
(b)
Table [1]
Significant figures [2]
Calculation of the average titre [2]
Titration consistency [3]
Agreement with supervisor’s titre [4]
[12]
NOTES
Table:
Table should include initial burette reading, final burette reading, and volume delivered.
The average titre should be calculated and the units included. Units missing (-1).
Significant figures:
All burette readings should be to at least one decimal place – each mistake is
penalised by 1 mark.
(However initial burette readings of 0 are penalised once only.)
If used, the second decimal place position should be 0 or 5 only – other values
will be penalised by 1 mark.
Average titre
Accurate titrations only should be used.
The use of a rough value is (-1).
The average value can be two decimal places, e.g. 25.37
An incorrect calculation is 0.
Mark denied if:
(i) only one accurate titration done
(ii) if titre not calculated correctly