Predictions for Chemistry 2014 HSC? (8 Viewers)

SuchSmallHands

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Yeah that's pretty good :) 4/4

You can also add in stuff lyk why high BOD is bad. eg a high BOD is indicative of a low dissolved oxygen content in the water because it is quickly used up by bacteria/organisms which use respire/use oxygen. This can lead to a eutrophication (containing nitrates and phosphates) and cyanobacteria which has a high BOD making the water unusable for any purposes.
Unpolluted water has BOD of less than 5 ppm and can be used.
I didn't mention eutrophication or cyanobacteria in my answer, would that still be able to get 4 if I'm only talking about drinking water/aquatic animals?
 

QZP

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I didn't mention eutrophication or cyanobacteria in my answer, would that still be able to get 4 if I'm only talking about drinking water/aquatic animals?
I liked your part about monitoring and I think it would achieve full marks. But not necessarily the BOD test (you missed the entire part about a control sample?)
 

enigma_1

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It's actually really sweet of you to underline key terms for us, thanks :)

BOD is measured using the BOD 5 day standard, which determines the amount (in milligrams) of oxygen consumed by one litre of water during 5 days in the dark at 20 degrees celsius. Dissolved oxygen (DO) is measured at the start and end of the period, with the difference between the two values being cited as the BOD. It is important to monitor BOD as it is an indication of the amount of organic waste which pollutes a waterway. A high BOD value indicates high levels of such pollution, and thus the water must be filtered before it is fit for human consumption. High levels of organic pollution, as indicated by a high BOD, can also cause waterways to become hypoxic, and thus not conducive to the lives of aquatic organisms. Thus, to preserve the habitats of such organisms it is important that BOD is monitored and and managed.
Heheh apologies I only just saw this now :)) But yeah haha no worries about that, just wanted to make it stand out more :) At the moment would be a 3/4

I didn't mention eutrophication or cyanobacteria in my answer, would that still be able to get 4 if I'm only talking about drinking water/aquatic animals?
Hmm yes I think if eutrophication was not mentioned, it would lose one mark (confirmed with my tutor previously) because BOD and eutrophication should always appear in the same answer. So yeah, adding in that extra word would also yield an extra mark :))

As QZP mentioned, that would also be necessary to add in, and then it would be ez 4/4 :)
 
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enigma_1

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Describe the nebulisation of the prepared solution and explain the need for the flame in AAS (3)
Oh lord is this the thing about ground state electrons and cation atomic emission spectroscopy etc? Hahah someone please answer, I'm terrible at this.
 

QZP

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I would give mine a 3/4 too because I didn't discuss the damages of very low/high BOD (which is part of the reason we must monitor BOD to ensure an optimal level)
 

SuchSmallHands

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Hmm yes I think if eutrophication was not mentioned, it would lose one mark (confirmed with my tutor previously) because BOD and eutrophication should always appear in the same answer. So yeah, adding in that extra word would also yield an extra mark :))
It's weird how we usually define BOD as an indicator of organic waste, but the presence of inorganic nitrate and phosphate ions is detected by it. Thanks for pointing that out! And thanks QZP too :)
 

enigma_1

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It's weird how we usually define BOD as an indicator of organic waste, but the presence of inorganic nitrate and phosphate ions is detected by it. Thanks for pointing that out! And thanks QZP too :)
Indeed! Oooh I have another question, alongside that of QZP's.
 

enigma_1

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Outline the Procedure of measuring the concentration of nitrates and phosphates in a water sample (3 marks)

[Hint: Kjedahl test]
 

timeflies

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Woah, I thought this thread had died because nobody posted for over a day and I log in now and there's like 5 new pages haha

Outline the Procedure of measuring the concentration of nitrates and phosphates in a water sample (3 marks)

[Hint: Kjedahl test]
Anyway, what is this test? I've never heard of it before this thread and someone mentioned it before as well. Do we need to know this? :(
 

SuchSmallHands

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Anyway, what is this test? I've never heard of it before this thread and someone mentioned it before as well. Do we need to know this? :(
I explained it last time someone brought it up (if you go back far enough you'll find my explanation of what you do, I can't remember who said they didn't understand but someone did and I described it), it's really no big deal since it's not mentioned directly in our syllabus. It just tests for nitrogen presence.
 

Hi there444

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Discuss the use of the suitable chemical to minimise acid or base spill. Use appropriate eqns. -5

_/_/_
 

enigma_1

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Outline the Procedure of measuring the concentration of nitrates and phosphates in a water sample (3 marks)

[Hint: Kjedahl test]
Woah, I thought this thread had died because nobody posted for over a day and I log in now and there's like 5 new pages haha



Anyway, what is this test? I've never heard of it before this thread and someone mentioned it before as well. Do we need to know this? :(
The nitrate concentration in water sample is measured using the Kjeldahl digestion method.
1) The water sample is boiled with concentration sulphuric acid to convert the nitrogen into ammonium sulphate.
2) It is then reacted with NaOH to form ammonia.

(NH4)2SO4(aq) + 2NaOH(aq) ------> Na2SO4(aq) + 2NH3(g)+ 2H2O (l)

3) Then, excess HCl of known concentration and volume (standard solution) is then added to the solution which is then back titrated to determine the concentration of nitrogen in the sample which is used to determine the nitrate concentration.

Furthermore, to determine/measure the concentration of phosphates, colorimetry is used.
1) A measured quantity of ammonium molybdate along with Nessler's reagent (catalyst) is added to the water sample and is mixed to form a yellow complex with phosphate known as phosphomolybbdate
2) Then ascorbic acid is added, reducing the intensity of the yellow complex to become an intense blue colour.
3) A colorimetry with the appropriate wavelength is selected and is shined through the sample.
4) The absorbance is calculated wwith a standard solution and the total phosphate concentration is determined since absorbance is proportional to concentration.

Then add this to make your answer look pretty and complete: nitrates and phosphates need to be monitored to prevent the incidence of algal blooms and cyanobacteria which can cause a high BOD of water, indicating eutrophication which makes the water unusable for any purposes.

Shouldn't take long, just memorise that
 
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iStudent

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State one environmental impact associated with the dry cell (1mk). This is a hsc q but I'm forcing you to choose dry over lead acid.
 

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