# Thread: HSC 2012-2015 Chemistry Marathon (archive)

1. ## Re: HSC 2012 Chemistry Marathon

Solution:
$n(BaSO_4) = \frac{1.8}{137.3+32.07+16 \times 4}$

$= 7.71 \times 10^{-3} moles$

$n(BaSO_4) = n(SO_4^{2-}) = 7.71 \times 10^{-3} moles$

$m(SO_4^{2-}) = n(SO_4^{2-}) \times (32.07+16 \times 4)$

$= 0.741g$

$(SO_4^{2-})\%(mass) = \frac{0.741}{2}$

$= 37\%$

Qn:

2. ## Re: HSC 2012 Chemistry Marathon

(a)
H2O(l)+CO2(g) <--> H2CO3(aq)
- Increasing the pressure within the system will favour the forward reaction as it would decrease pressure within the system.
- If the forward reaction is favoured, an increased amount gas is dissolved into solution.

(b)
The forward reaction of "H2O(l)+CO2(g) <--> H2CO3(aq)" is exothermic so a lower temperatures, the reaction favours the direction that increases temperature - the forward reaction and so more gas will be dissolved.

Consider the following equation for the reaction of potassium dichromate with an acidified solution of sodium iodide.
Cr2O7-2(aq) + 14H+(aq) +6I-(aq) 2Cr3+(aq)+ 3I2(aq) + 7H2O(l)
What is the oxidation state of chromium in the dichromate ion?
Identify which products have been reduced and which have been oxidised.
Justify the omission of sodium and potassium from the ionic equation above in terms of movements of electrons.

3. ## Re: HSC 2012 Chemistry Marathon

What exactly is "titration volume"?

Also, I don't see why this line, "n(Na2CO3)= 0.05404..... x 0.02235 = 0.001207......" works. How does that show the number of moles that reacted.

4. ## Re: HSC 2012 Chemistry Marathon

Originally Posted by someth1ng
What exactly is "titration volume"?

Also, I don't see why this line, "n(Na2CO3)= 0.05404..... x 0.02235 = 0.001207......" works. How does that show the number of moles that reacted.
titration volume in that question refers to the amount of sodium carbonate needed to neutralise/reach the end point the HCl.
So thus 22.35mL of sodium carbonate was needed to neutralise 25mL of HCl.

Using that knowledge, we can then determine the no. of mols of sodium carbonate needed to neutralise the HCl, which is found by n = C x V = 0.05404 (conc. from part 1) x 0.02235 (volume) = 0.001207 mols.

5. ## Re: HSC 2012 Chemistry Marathon

Originally Posted by Dylanamali
titration volume in that question refers to the amount of sodium carbonate needed to neutralise/reach the end point the HCl.
So thus 22.35mL of sodium carbonate was needed to neutralise 25mL of HCl.
I was probably confused since I didn't have the definition of "titration volume" in my mind.

Originally Posted by Dylanamali
titration volume in that question refers to the amount of sodium carbonate needed to neutralise/reach the end point the HCl.
So thus 22.35mL of sodium carbonate was needed to neutralise 25mL of HCl.
Why is it 22.35mL Na2CO3:25mL HCl and not the 250mL Na2CO3:22.35mL HCl?

Originally Posted by Dubble-u25
c(HCl)=0.001207...../0.025 = 0.04831 (4sig.figs.)
Why is it divided by 0.025L if only 0.02235L reacted?

To clear things up:
I was thinking, why isn't it how much HCl is needed to neutralise the Na2CO3?

After reading the question again:
"Four 25.00mL samples of acid were titrated with the sodium carbonate solution. The average titration volume was 22.35mL. "

Does this mean: there were four flasks of 25mL of HCl and in each, they put 22.35mL of the Sodium Carbonate solution?

6. ## Re: HSC 2012 Chemistry Marathon

Have you executed the titration prac yet at school? If not, then it might be difficult to understand.

But yeah, in that question - a 250mL solution of sodium carbonate was made with a concentration of 0.05404 M as easily determined in part 1.
This solution is then poured into the burette. Whilst 25mL of the acid is pipetted into the conical flask.
The burette is then used to pour the sodium carbonate solution into the conical flask containing the acid (it also contains an indicator - most likely methyl orange in this case - ask if you want to know why its methyl orange). The methyl orange indicator changed colour once 22.35mL of the sodium carbonate base had been added to the acid - indicating the equivalence point and point of neutralisation. That is why its 22.35mL of sodium carbonate not 250mL - because only 22.35mL of sodium carbonate was buretted into the flask not 250.

Firstly, we used the titration volume and multiplied that by the concentration of the sodium carbonate to find the no. of mols of sodium carbonate. We then apply molar ratios, in this case 1:2 sodium carbonate:HCl, to find the no. of mols of HCl that were in the flask. We found this to be 0.001207x2 mols. After this to find the concentration of HCl we use C = n/V.
In this case the volume is 0.025 and not 0.02235 because the 22.35mL was indicative of the sodium carbonate needed, but in this calculation we are determining the conc of the HCl - and we know from the question that 25mL of HCl was pipetted into the flasks.

7. ## Re: HSC 2012 Chemistry Marathon

Yeah, I read your response then read the question then realised that I didn't read it properly. Four 25.00mL samples of acid were titrated with the sodium carbonate solution.
I thought it meant the Sodium Carbonate solution was titrated with HCl so I was so lost! haha

It all makes perfect sense now - thanks guys.

In the HSC, will they follow you through solving it? Like, can it ask you somthing like:

"A standard solution was prepared by dissolving 1.432g of sodium carbonate in water. The solution was made up to 250mL in a volumetric flask. Four 25.00mL samples of acid were titrated with the sodium carbonate solution. The average titration volume was 22.35mL.

Calculate the concentration of the hydrochloric acid solution."

8. ## Re: HSC 2012 Chemistry Marathon

Originally Posted by someth1ng
Does this mean: there were four flasks of 25mL of HCl and in each, they put 22.35mL of the Sodium Carbonate solution?
Not neccessarily.. it probably means that they had four flasks of 25mL of HCl and performed 4 titrations.
The first being a rough titration to gain an approximate indication of where the eq. point is and next 3 being more accurate. The would then discard the rough titration value and use the other 3 values to determine a titration volume average - which was 22.35mL of Na2CO3.

9. ## Re: HSC 2012 Chemistry Marathon

Originally Posted by someth1ng
In the HSC, will they follow you through solving it? Like, can it ask you somthing like:

"A standard solution was prepared by dissolving 1.432g of sodium carbonate in water. The solution was made up to 250mL in a volumetric flask. Four 25.00mL samples of acid were titrated with the sodium carbonate solution. The average titration volume was 22.35mL.

Calculate the concentration of the hydrochloric acid solution."
Yes - generally with titration questions in the HSC they are split up into 2 or 3 parts. So they get you to break down each component of the question.
a) find the concentration of the sodium carbonate solution (1 mark)
b) find the concentration of the HCl solution (2 marks)

10. ## Re: HSC 2012 Chemistry Marathon

Explain why Sodium Hydorxide is not suitable to be used as a primary standard and justify your answer (3 marks)
Define Le Chatlier's principle and outline the factors it effects.
Analyse how neutralisation reactions can be used to minimise damages during chemical spills.
What is condensation polymerisation?
Describe the process for the production of Hight density and Low density Polyethylene, and compare and contrast its uses with its properties (6 marks)

More questions coming up LOL

11. ## Re: HSC 2012 Chemistry Marathon

You have performed a practical on Esterfication. Outline and steps your undertook and assess the reliability, validity and accuracy of the experiment.
Define pH? (Simple q)
Discuss why the pipette and burette should be rinsed with the solution they will contain before conducting the titration.

12. ## Re: HSC 2012 Chemistry Marathon

btw - just out of curiosity where is everyone at in the syllabus in HSC chem atm? how far into acidic are you guys?

^ sanjoy you already done esterification at school?? =O

13. ## Re: HSC 2012 Chemistry Marathon

Originally Posted by Dylanamali
btw - just out of curiosity where is everyone at in the syllabus in HSC chem atm? how far into acidic are you guys?

^ sanjoy you already done esterification at school?? =O
We started with the acidic environment, and we are up to the 3rd context point of Production of materials. We'll probably finish POM soon, and start Monitoring chemical... Or revise for Half yearlies

14. ## Re: HSC 2012 Chemistry Marathon

ohh right, no wonder.

ah k fair enough.. yeah I wouldn't think that you'd begin monitoring before half yearlies, you'll probs do revision.

15. ## Re: HSC 2012 Chemistry Marathon

1 more thing about the titration, isn't there 2 moles of HCl per 1 mole Na2CO3? Hence, you need to double that final concentration?

After doing that, I got 0.09662 mol/L

I'm only early into the Acidic Environment - end of PFA 2 and start of PFA 3.

16. ## Re: HSC 2012 Chemistry Marathon

Na2CO3 + 2HCl ---> 2NaCl + H2O + CO2

that's the equation. so yeah your right.. my bad, I wasn't looking at the question properly and only read off the answers.
so yeah that's correct.

So you haven't even learnt titrations yet? just read ahead, I presume?

17. ## Re: HSC 2012 Chemistry Marathon

Originally Posted by Dylanamali
Na2CO3 + 2HCl ---> 2NaCl + H2O + CO2

that's the equation. so yeah your right.. my bad, I wasn't looking at the question properly and only read off the answers.
so yeah that's correct.

So you haven't even learnt titrations yet? just read ahead, I presume?
Yeah, no biggie.

Our school hasn't done titrations yet but they give us 20 questions and 10 questions will be asked in the half yearly (calculations have different values) - I just wanted to know all the solutions so I could get all the notes ready since I know pretty much everything else already. The actual working doesn't seem so complicated after you understand what the question is asking and understanding all the jargon.

Time for me to go to sleep - my bed time is well overdue.
Goodnight, thanks for your help.

18. ## Re: HSC 2012 Chemistry Marathon

ah right.. that's preety good then lol.
you can preety much prep yourself to get 100%

19. ## Re: HSC 2012 Chemistry Marathon

Originally Posted by Dylanamali
btw - just out of curiosity where is everyone at in the syllabus in HSC chem atm? how far into acidic are you guys?

^ sanjoy you already done esterification at school?? =O
We've finished production of materials but we're only up to the second dot point in acidic so I haven't got a clue about all these titrations lol

20. ## Re: HSC 2012 Chemistry Marathon

Originally Posted by Kimyia
We've finished production of materials but we're only up to the second dot point in acidic so I haven't got a clue about all these titrations lol
ahha yeah I wouldn't expect many schools to have done titrations at this stage..
probably a decent place to be at right now would be just finishing off Le Chatelier's principle and getting into the theory behind acids and bases - in terms of strength, concentration etc.

21. ## Re: HSC 2012 Chemistry Marathon

Originally Posted by Dylanamali
ahha yeah I wouldn't expect many schools to have done titrations at this stage..
probably a decent place to be at right now would be just finishing off Le Chatelier's principle and getting into the theory behind acids and bases - in terms of strength, concentration etc.
We're roughly around there.

22. ## Re: HSC 2012 Chemistry Marathon

New question:

23. ## Re: HSC 2012 Chemistry Marathon

what is the temperature and pressure in which high density polyethylene is made? (confused bcos in excel it says 'just above 300 celsius' whereas in KISS booklet it says at about 60 celsius..which ones correct..can it b a typo

24. ## Re: HSC 2012 Chemistry Marathon

Originally Posted by nightweaver066
New question:
The properties of Technetium 99m make it very useful for its applications as a medical tracer in medicine. As it undergoes gamma decay (maybe write equation, but its only 3 marks), it can be placed inside the body effectively 'tagging' the required area. For example, it can be used to detect blood clots and brain tumours. Technetium 99m can also be combined with other compounds to alter its properties, extending its usefullness. Combining it with tin causes it to attached to red blood cells, allowing examination of the heart and blood vessels.

Its short half life of only 6 hours minimises a patients exposure to radiation, making it quite safe to use. In addition, it is not used naturally by any part of the body so it isn't abosrbed, meaning that it can be used to examine radiation sensitive areas such as the heart and kidneys.

Its short half life however also means that it must be produced near where it is used, potentially limiting its usefullness. On balance however, the properties of Technetium 99m make it very useful.

In a test I would probably write that better/more concisely but those are the points I would want to cover...

Now a big marker:
Evaluate the impact of industrial sources of sulfur dioxide and nitrogen oxides on the environment, making use of appropriate chemical equations. (7)

25. ## Re: HSC 2012 Chemistry Marathon

Originally Posted by viraj30
what is the temperature and pressure in which high density polyethylene is made? (confused bcos in excel it says 'just above 300 celsius' whereas in KISS booklet it says at about 60 celsius..which ones correct..can it b a typo
LDPE is conducted in an environment with pressure equivalent to approximately 3000 atmospheres, and with extremely high temperatures. The high pressure atmosphere leads to haphazard branching in the propagation process, leading to the low density. HDPE on the other hand is conducted using the Ziggler-Natta process. Pressure equivalent to only a few atmospheres is required with low temperatures, due to the catalyst present. This low pressure results in a tightly packed, high density polymer.

Right/Wrong?

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