HSC Chemistry Question Thread

[goodsj]

here's a flowchart from the success one hsc chem book:
sorry if you've already seen this flowchart before when you mentioned you've looked at different resources, but I think this flowchart covers everything pretty well
According to marker's comments: flowchart should be clear and comprehensive, showing processes and conditions.  Flowcharts showed a number of steps, placed in the correct sequence, identified physical and chemical processes, including appropriate catalysts

[img width=2000 height=1500]http://i.imgur.com/WQDGv6a.jpg[/img]

Thankyou!

[skullcandy]

Hey Guys,

Could any of you help me answer these two questions:


For question 9, I believe the atoms may tend to cool as the H20 would reach equilibrium

Thanks <3

[JuliaPascale123]

I feel retarted as I cant answer this..

[kemi]

I feel retarted as I cant answer this..
(Image removed from quote.)

Given the mass, you can find the no. of moles of K2O:

n = m/M
n(K2O) = 100/[2(39.10)+16]
              = 1.061571125...

But the no. of moles of potassium is actually twice the number of moles of potassum oxide (two potassium ions per molecule)

Hence,

n(K) = 2 x 1.061571125...
        = 2.123142251...

Now we apply the formula

n = N/NA, where N is the number of particles, and NA is Avogadro's constant (6.022 x 10^23)

Rearrange:

N = n x NA
N(K) = 2.123142251... x (6.022 x 10^23)
         = 1.278556263... x 10^24
         = 1.28 x 10^24 (3.s.f.)

Therefore, there are 1.28 x 10^24 potassium ions in the sample.

[winstondarmawan]

Hello! Can someone please go over the advantages and disadvantages of scientific models?
TIA.

[angelahchan]

Hey Guys,

Could any of you help me answer these two questions:
(Image removed from quote.)

For question 9, I believe the atoms may tend to cool as the H20 would reach equilibrium

Thanks <3

For Q9 I think you're meant to talk about Le Chatelier's principle. e.g. Increase in temp would shift equilibrium to the right favouring the forward reaction since it's endothermic, as predicted by le chat's principle system in equilibrium shifts to counteract change, thus the shift would increase amount of CO and H2O produced.

Hello! Can someone please go over the advantages and disadvantages of scientific models?
TIA.

Advantages: allows us to observe a concept we can't see with the naked eye/difficult to show in school lab, easy to understand complex concepts with visual representation
Disadvantages: oversimplifies processes, you can say it misses out a certain part of the thing it's representing  (I know this sounds really vague, but it depends on what model you're talking about- there's usually always something the model skips out on and you can pretty much make it up)

[JuliaPascale123]

Hey Guys,

Could any of you help me answer these two questions:
(Image removed from quote.)

For question 9, I believe the atoms may tend to cool as the H20 would reach equilibrium

Thanks <3

This looks like a really good question anyone know how to answer it?

[toasted]

Hey!!!

I'm really struggling to find information about the following dotpoints from the syllabus- they're under the option shipwrecks:
-Identify the use of electrolysis as a means of cleaning and stabilising iron, copper and lead artefacts
-Discuss the range of chemical procedures which can be used to clean, preserve and stabilize artefacts from wrecks and, where possible, provide an example of the use of each procedure

I've looked through textbooks and they're all so brief- so im unsure if there's any more info on them. Anything would be amazing 

[clarence.harre]

Can someone please explain responding in significant figures?
To expand:
I don't mean how to answer in sig fig, but when to answer in sig fig and to how many we're meant to do.
My understanding so far is the smallest number of significant figures in the question is the number of sig fig we're meant to answer to, but often I'll do that and be marked wrong because my rounded answer isn't what the teacher has on their marking sheet - although I do put the non-rounded answer before it i.e. Answer = 1.4356623.... = 1.44 (3 sig fig)
It's just extremely irritating that the marking guidelines sometimes specify sig fig and if I don't do it then I'll be marked down, so I've just been doing it for every question, but it seems that if it isn't specified then for some reason I'm also marked down.
Calculations are correct though. Maybe my teachers are just religious when it comes to marking.

[clarence.harre]

Hey!!!

I'm really struggling to find information about the following dotpoints from the syllabus- they're under the option shipwrecks:
-Identify the use of electrolysis as a means of cleaning and stabilising iron, copper and lead artefacts
-Discuss the range of chemical procedures which can be used to clean, preserve and stabilize artefacts from wrecks and, where possible, provide an example of the use of each procedure

I've looked through textbooks and they're all so brief- so im unsure if there's any more info on them. Anything would be amazing 

Identify the use of electrolysis as a means of cleaning and stabilising iron, copper and lead artefacts

If a question asks how to clean or stabilise any metallic artefact, you just need to go "oh yeah, electrolysis".
Pretty much you just make the metal artefact the cathode by attaching it to the negative terminal and an inert (must be inert) steel electrode as the anode. Place it in a dilute alkaline solution (i.e. 2% NaOH) and pump a small current through it. This will draw out the chlorine and sulfate ions, effectively cleaning and stabilising it. If you want to get rid of the concretions, pump a high current through it. You'll know it's working when you get hydrogen gas bubbling up near the cathode. The concretions should break away.

Discuss the range of chemical procedures which can be used to clean, preserve and stabilize artefacts from wrecks and, where possible, provide an example of the use of each procedure

I take it this refers to the process of finding something, going 'yep, we gotta fix that'.

1. First would be getting rid of anything keeping it submerged, using a drill or in the case of the Endeavour Canons, using explosives.
2. Now we pack up the artefact, keeping it wet with water or sand from the environment it was found in - this stops it drying out. If it dries out the salts saturated inside will expand into their crystal form and damage both the structural integrity and the aesthetic of the artefact. It is sent to the lab for cleaning and such
3. Remove the concretions (calcium carbonate) using either a hammer, electrolysis or acid
Example: Hammer was used in the Endeavour Canons
4. Get rid of salts using electrolysis (the first point you talked about). Constantly measure the chloride concentration in the solution. When the it seems to be plataeuing, replace the solution with a new one. Keep doing this until the concetration reaches 25ppm (some sources say as high as 50ppm, but less is always better for the artefact).
Example: The Sirius anchor spent 8 weeks doing it (well that's what the guy at the national maritime museum said)
5. Coat the artefact - either clear polyurethane polymer or micro crystalline wax
Example: Verner Anchor - displayed outside the National Maritime Museum
Fun fact - kids kept playing on them and eventually rubbed the coating off. So they'd been corroding again for a bit until people realised what was going on.
As they're also an outdoor 24/7 display item, meaning they are exposed to heaps of oxygen and water and all the good stuff that accelerates corrosion, they stand on a metallic mesh that acts as a sacrificial anode.

Think that's about it. If I've missed anything, definitely add to the list, correct my errors (bet there's at least 3) and expand on points.
Hope it helps

[kemi]

Can someone please explain responding in significant figures?
To expand:
I don't mean how to answer in sig fig, but when to answer in sig fig and to how many we're meant to do.
My understanding so far is the smallest number of significant figures in the question is the number of sig fig we're meant to answer to, but often I'll do that and be marked wrong because my rounded answer isn't what the teacher has on their marking sheet - although I do put the non-rounded answer before it i.e. Answer = 1.4356623.... = 1.44 (3 sig fig)
It's just extremely irritating that the marking guidelines sometimes specify sig fig and if I don't do it then I'll be marked down, so I've just been doing it for every question, but it seems that if it isn't specified then for some reason I'm also marked down.
Calculations are correct though. Maybe my teachers are just religious when it comes to marking.

What you're doing is correct - always round to sig fig, because in the HSC there is ONE question that tests students on sig fig, but nobody knows which one it is. So just play it safe - round to the smallest number of sig figs, as given in the question.

[michelleh]

Hi guys!

Just a quesstion: Is the radioisotope, Cobalt-60, used to detect pipe leakages? CAN they detect pipe leakages (e,g flushed through pipes and emits gamma radiation which is picked up by a gamma detector/camera etc.)?

Thanks everyone!

[Natasha.97]

Hi guys!

Just a quesstion: Is the radioisotope, Cobalt-60, used to detect pipe leakages? CAN they detect pipe leakages (e,g flushed through pipes and emits gamma radiation which is picked up by a gamma detector/camera etc.)?

Thanks everyone!

Yes, Co-60 can be used to detect cracks/breaks in pipes

[bsdfjnlkasn]

Hey there!

I know this has been answered already, but any ideas on how to solve this? Wouldn't mind getting linked to the place to save someone the time of solving it (again)

[kauac]

Hi, this is a question for the biochemistry of movement module, though it kind of relates to the other modules in terms of naming of compounds.

What are the differences in the names used for lactic acid - specifically, 2-hydroxypropanoic acid and 2-hydroxypropionic acid?? The first one is the IUPAC name, though I can't seem to find any information on the second one, and what its purpose is.

Any help greatly appreciated. Thanks!