THE BIG ADVICE:
You need to get in the
mindset and be hyperaware that that you need to distinguish yourself. No matter
how much you know if you don’t include detail, address the verb, define terms
etc, there is no way of distinguishing you from someone who knows nothing. Ask
yourself – could someone who was trying to ‘wing it’ write my answer? What have
I added that cannot be ‘winged’ that makes me stand out?
2001
Q13) Note –
Barium sulphate ppt forms almost immediately – waiting will not make more
precipitate
26a) Make your data points stand out. Not just tiny dots
(but not huge blobs either)
Q15) The
solubility rules are very simple and we have used them since yr 11. I am
disappointed that anyone got this question wrong. And considering that our
flowchart test for lead is to add chloride ions those who put A and D really
need t ask if they are taking ownership of their learning in this subject.
2002
Q24) If asked
to assess the impact of AAS always give an illustrative example/case study.
Also note that it was more accurate than previous methods and say ‘eg gravimetric
analysis’ thus could determine previously unknown roles of trace elements. Also
always define key terms in questions. In this Q you had to define ‘trace
element’. – NB because lead has NO biological function it is NOT a trace
element. Being an assess you should have also had a judgement.
2003
27b) DEFINE
KEY TERMS – Reliability – repeat with consistent results – was it repeated?...
THEN go on to talk about other errors – excess Ba added?, acid dissolution?,
washed and dried?. EVALUATE NEEDS A JUDGEMENT! If you don’t talk about
reliability/repetition and just talk about error you are assessing
validity/accuracy. Also – sulfates will ppt in all pH’s, it is only carbonates
and phosphates that don’t ppt at low pH – hence low pH is idea if you only want
to ppt sulfates.
28) Use a
ruler to draw ONE straight line of best fit. And USE your graph to calculate
concentrations. The whole point of AAS is accuracy. You don’t buy and multi million
S AAS machine, go through the process of making an accurate calibration curve
and testing your samples only to use your eye and make a rough guess about he
concentration “About 5ppm”… To prove it was Sewage you have to state what the
Cd was upstream and downstream of it and somehow show it cant be from the
industrial plant.
2004
20) This is a critical Q testing your understanding of
AAS. The light source contains the metal being tested to produce the exact
wavelength of light that will be absorbed by the sample (to increase accuracy
and NB% absorbance proportional to conc). The flame atomises the sample o the
atoms CAN absorb those unique wavelengths of its absorption spectrum (remember
element only absorb emit their unique spectra when atomised).
20b) Again –
define validity and reliability. Is testing from one sample (even if it is
tested 5 time) really a reliable way of analysing a whole industrial site? Did
they calculate the absorbance in a valid manner? Is AS itself a valid and
reliable and accurate technique?
20c) If you
write NH3 for the phosphate test give yourself a facepalm then go back and
check our flowchart an flowchart equations VERY carefully. IF you used Ba io ns
for both phosphate and sulphate without distinguishing them give yourself a
facepalm (other hand). The whole point of ppt tests is to be unambiguous you
can have a white Ba ppt signifying both sulfate ad phosphate unless you sy that
the phosphate dissolves in acid but the sulphate ppt does not.
2005
Q25b) A
chemical test = a precipitation test
(AAS is a physical test as it involves no chemical reactions). And while Cl- is
OK a better one would be to add I- as it gives the distinctive ‘canary yellow’
ppt.
Q25c) The
statement said ‘concentrations’ so your answer should have given an example
concentration. And you justify the need to monitor BY showing how bad it is if
it is in the body – ie it needs to be monitored to avoid these problems.
2006
Q25) You had
to make estimates of concentration from the graph AND concentrations need units
– ie ppm! Also – don’t say ‘approximately’ unless you know the data is very
unreliable– it is AAS data – ay your results with conviction!
Q26b) To
explain the order you should also say what would happen if the correct order
was not used eg add Ag first and get a white ppt which could mean….?
2007
Q14) When
electrons ‘fall back’ they EMIT light not absorb light and this is absorption
spectroscopy. The light is NOT white light, it is the light from the spectral
lamp.
17a) No
matter how far you are through the course ALWAYS use the cross method to make
sure your ionic formulae are correct. States
DO matter. If they are solutions the reactants must be aqueous. In a one
mark ‘write the equation’ question you will get ZERO if one state is wrong.
19b) Not much
working was shown for this Q – very suspicious. If not showing mathematical
working you need to show your explanation – ie X mg/L but the tablet was only
in 100ml. Thus the mass would have been 1/10th this value = 0.X mg
in the tablet.
2009
Q24) This was
a big Q for 5 marks, You needed to briefly intro the fact the elements have
unique spectra, you needed to BRIEFLY outline the AAS process lamp, furnace, monochromator,
detector, (WITH A DIAGRAM) CRITICAL to
note that the % of light absorbed is proportional to conc. outline the
calibration curve then ID the fact that
it is accurate then give an example of environmental monitoring – and NB the
trace element ones are OK for environmental monitoring but ‘environmental
monitoring’ in the HSC context usually means pollution – ie lead. If I was
doing the HSC this is one of those Q that I would practise a few times as it
covers the whole range of AAs concepts in one question.
2010
29a) Broken
record time – DEFINE VALIDITY FIRST. All the SULFATE in the fertiliser was
dissolved (NB they filter out the solids before adding Ba so clearly not all
the fertilizer dissolved – but we only care that the sulphate dissolved). All
the sulphate was ppt by excess Ba ions. All the ppt remained in the filter
paper. The ppt and paper was fully rinsed. The ppt and paper was dried to
constant mass. That no carbonate or phosphate ppts formed with Ba. Also –
regardless of *that* years marking scheme – for three marks they could expect
three validity issues.
2012
Q32) This is an excellent test of your data
analysis and calculations skills. A LOT of solutions matched the Success One
book very closely. THIS IS FINE as long as you are learning. This is another
question that I would come back to a number o times to make sure I understood
how to answer this type of question. Ie come back to it and try again without
looking at your previous answer.
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