Practical Assessment Marking Scheme on General Errors:
Additional comments – 2008
Additional comments - 2009
Each student performed two techniques. They may not be two complete techniques, but
they must have 5 checks on each technique. For whatever reason if they don’t have 5
checks for a technique, that incomplete technique is still out of 15 (=3x5).
In three cases, students took a long time with one technique. Coincidentally, they were all
really slow in doing the calculation for Technique 3 – Use of Volumetric Flask. So, they
ran out of time. But Technique 3 has a lot of Technique 2 in there. So, I marked them on
Technique 2 instead of giving them zero for not completing Technique 7. So, three
students were marked on this combination: Technique 3 and Technique 2 combination.
Below are the common mistakes that I came across in Sets A and C. To be consistent,
here is how I graded them whenever these common mistakes were observed.
1. Goggles: If I had to remind them to wear goggles, -1 mark. when mark is
converted to be out of 20.
2. Record in pencil: -1 mark out of 20.
3. Keywords:
Know keyword, but cannot pronounce it – G
Each mistake, down grade by 1
3. Acclimatization:
Acclimatize once – S
Acclimatize twice – G
Acclimatize or say they will do it 3X – E
Contamination - When they are sucking up the solution during
acclimatization and the liquid that is being drawn into
the pipette is going up and down, the solution is
contaminated. If the same solution is used as the sample
solution – S
Not enough solution used in acclimatization. Solution should be drawn to
at least 1/3 in the bulb – S
4. Calculation:
Calculation correct but incorrect sig figs - S
Incorrect/no units – S
Improper sig fig from calculation - S
5. Technique 1 and 8: Burette:
Angled on the burette stand - S
Wet outside – S
Burette tip not well inside the neck of the Erlenmeyer flask – G
Take burette off the clamp to read – G
Bubble remains even after trying to dislodge it – S
Does not know bubble is there or check for bubble – U
Disagreement with reading – U
Record to improper sig figs (not from calculations) – U
Pipette from reagent bottle – U
6. Technique 2 - Pipetting:
If they wipe pipette tip then drain liquid to mark – E
If they drain liquid to mark then wipe pipette tip – G
Draining pipette:
for 5 sec or less – U
between 5 to 8 sec – S
between 8-9 sec – G
for 10 sec – E
Does not touch side when count to 10 – U
Forget to replace cap on reagent bottle – S
Drain with tip of pipette immersed in solution – U
Level of solution fall below or over mark – S
Level of solution at the mark but a drop hangs outside the tip of the
pipette. Does not remove the hanging drop. – S
Use paper towel to wipe – S
Dispense into a beaker too small – S
Use thumb instead of index finger to control the solution – S
Use big beaker for pipetting out of – S
Record to improper sig figs (not from calculations) – U
Pipette from reagent bottle - U
7. Technique 3 and 7: Volumetric Flask:
Over or under the mark – U
Use water bottle to bring to mark – S
Forget to replace cap on reagent bottle – S
Improper mixing (not inverting 15 times) – S
Use Pasteur pipette directly from water bottle – S
Record to improper sig figs (not from calculations) – U
Pipette from reagent bottle - U
8. Technique 4 – Weighing
If the mass of sand is < 0.4 g or > 0.6 g – S
Did not clean, or if they clean/sweep the balance chamber while balance is
‘ON’ – S
Forget to replace cap on reagent bottle – S
If they didn’t wait for the ‘g’ to appear and they took a reading – U
Instead of turning balance ‘OFF’, turned to CALIBRATE – S
If excess sand/compound is scooped back into the bottle – U
Door open when weighing – S to U
Zero balance with door open – U
Does not write data in the datasheet – U to S
Record to improper sig figs (not from calculations) – U
9. Technique 5 – Bottle-Top Dispenser
Lift graduated cylinder off the surface or remove burette from clamp to
take a reading – S
Dispense into beaker then transfer to the graduated cylinder – G
Pressed on the plunger while dispensing – S
Didn’t clear air bubble prior to measuring step – S
Record to improper sig figs (not from calculations) – U
10. Technique 6 - Incorrect folding of filter paper – U
Washing with a constant stream of water - S
Place filter paper on bench to fold - S
Decanting - Does not direct rod over thick part of the filter paper - G
Does not use the rubber part of the rubber policeman during scraping and
transferring – S
Place rubber policeman down without rinsing - U
11. Technique 7 – Weighing and Standard Solution Preparation
Quantitative transfer:
Rinse once – S
Rinse twice – G
Rinse or say they will do it 3X – E
Does not dissolve crystal in the beaker before transferring to flask – S
Record to improper sig figs (not from calculations) – U
12. Technique 8 - Titration:
If didn’t choose to use the small beaker to pour acid into – S
If the second titration volume is within +/- 0.20 mL of Viara’s titration
volume – G
If the second titration volume is +/- within 0.30 mL of Viara’s titration
volume - S
If the second titration volume is > +/- 0.30 mL of Viara’s titration volume
-U
No dispensing quickly close to the 1st titration – S
Half-drop technique – drop by 1 for each missed try
No white paper under the Erlenmeyer flask – S
Disagreement with reading – U
Record to improper sig figs (not from calculations) – U
Pipette from reagent bottle - U