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September 24, 2020, 10:58:31 am

Author Topic: VCE Chemistry Question Thread  (Read 1260398 times)  Share 

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zhouzhennan

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Re: VCE Chemistry Question Thread
« Reply #8670 on: July 29, 2020, 06:58:32 pm »
+1
Hey there im wondering how relevant is pH and acid and base stuff(and indicator) in Year 12 VCE? Is it possible that there will be questions on them in the exam?

colline

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Re: VCE Chemistry Question Thread
« Reply #8671 on: July 30, 2020, 08:04:24 am »
+5
Hey there im wondering how relevant is pH and acid and base stuff(and indicator) in Year 12 VCE? Is it possible that there will be questions on them in the exam?

Nope, they're not on the 3/4 study design. Doesn't hurt to still learn it though ;)

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Coolgalbornin03Lo

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Re: VCE Chemistry Question Thread
« Reply #8672 on: August 01, 2020, 09:13:22 am »
+1
1. In a chromatogram is it peak height or peak area which tells you about concentration of components? I thought it was area but now Iím not so sure.....

2. And is it necessary to know about the theory of NMR and IR? I know about nucleons spinning and changing the vibrational energy of covalent bonds but anything else?
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IThinkIFailed

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Re: VCE Chemistry Question Thread
« Reply #8673 on: August 01, 2020, 10:23:00 am »
+4
1. In a chromatogram is it peak height or peak area which tells you about concentration of components? I thought it was area but now Iím not so sure.....

2. And is it necessary to know about the theory of NMR and IR? I know about nucleons spinning and changing the vibrational energy of covalent bonds but anything else?

1: Peak area tells you about the concentration of components. Generally though, youíd need to compare the peak areas to peak areas produced by standard solutions ran through the same conditions to determine concentration.

2:
Yea I donít know about this either lol, the study design is kinda vague and just says ďgeneral principlesĒ, but it does state specifically you need to know about spin energy levels regarding NMR.
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1729

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Re: VCE Chemistry Question Thread
« Reply #8674 on: August 01, 2020, 10:30:38 am »
+2
1. In a chromatogram is it peak height or peak area which tells you about concentration of components? I thought it was area but now Iím not so sure.....
IThinkIFailed already replied. But I would just like to add area can give you a relationship about the concentration it can't tell you the concentration directly, but here's some example HPLC data I collected for a lab.


sweetcheeks

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Re: VCE Chemistry Question Thread
« Reply #8675 on: August 01, 2020, 12:51:32 pm »
+5
For HPLC both peak height and peak area are able to be used for concentration. Peak area is generally better, as the area doesn't vary as much as height does if parameters slightly change during the run (slight fluctuations in flow rate, temperature etc. ). Sometimes peak height needs to be used if there are two species that elute close to each-other (in this case, the peak area will be overlapping between the two and differentiating the two is nearly impossible).

Of course, you need some sort of reference to compare to. For the purpose of VCE, you just need to know about the calibration curve method.

Coolgalbornin03Lo

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Re: VCE Chemistry Question Thread
« Reply #8676 on: August 01, 2020, 08:37:26 pm »
0
1. Because isomers are the same compound does that mean their fingerprint regions are the same?

2. Why does decreasing particle size of stationary phase increase retention time? I thought because SA has increased THERES more chances for the components to adsorb and desorb quicker into the mobile phase?

Sorry for posting so much today!
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sweetcheeks

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Re: VCE Chemistry Question Thread
« Reply #8677 on: August 01, 2020, 10:47:04 pm »
+4
1. Because isomers are the same compound does that mean their fingerprint regions are the same?

2. Why does decreasing particle size of stationary phase increase retention time? I thought because SA has increased THERES more chances for the components to adsorb and desorb quicker into the mobile phase?

Sorry for posting so much today!

1. Do you mean isomers or enantiomers? If you mean enantiomers, than yes, their fingerprint regions will be the same. For isomeric species (those that are not enantiomers), their fingerprint regions will differ.

2. I'm not sure why you think it would be quicker. Molecules are initially in the mobile phase and as they move through the column they will adsorb to the stationary phase, desorb back into the mobile phase and continue this cycle throughout the column. If you have a higher surface area, there is more chance for these molecules to absorb onto the stationary phase and this will increase the overall time they are in the stationary phase, resulting in them remaining in the column longer. You can assume that each adsorption/desorption process takes the same amount of time, so increasing the amount of events will increase the amount of time the molecule spends in the column.

Coolgalbornin03Lo

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Re: VCE Chemistry Question Thread
« Reply #8678 on: August 03, 2020, 09:39:49 pm »
0
Why is the volume of [insert standard solution] added to neutralise (reach equivalence point) the same for a strong base and weak acid? I thought the strong acid would need more as itís pH is much lower?
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Re: VCE Chemistry Question Thread
« Reply #8679 on: August 03, 2020, 11:20:04 pm »
+2
Why is the volume of [insert standard solution] added to neutralise (reach equivalence point) the same for a strong base and weak acid? I thought the strong acid would need more as itís pH is much lower?
Strong acid and weak acid, assuming all other factors constant, still have the same moles of acid. The only difference is how much of the acid in question is dissociated pH is a measure of the acid dissociation in a solution, and since weak acids dissociate less, their recorded pH is higher in a solution compared to an identical strong acid solution.

Coolgalbornin03Lo

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Re: VCE Chemistry Question Thread
« Reply #8680 on: August 05, 2020, 08:55:11 pm »
0
Why is NaOH not a good primary standard? I know it reacts readily with oxygen and CO2 in atmosphere but the solutions also say it needs to have a known Formula mass? I know this is a requirement for a primary standard but isnít sodium hydroxides formula mass already known.
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keltingmeith

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Re: VCE Chemistry Question Thread
« Reply #8681 on: August 05, 2020, 09:42:11 pm »
+4
Why is NaOH not a good primary standard? I know it reacts readily with oxygen and CO2 in atmosphere but the solutions also say it needs to have a known Formula mass? I know this is a requirement for a primary standard but isnít sodium hydroxides formula mass already known.

It would have a known formula mass if all of it was NaOH, but as you said - NaOH reacts with oxygen and CO2. So some of it reacts with oxygen to make sodium oxide and water, and the CO2 will make sodium carbonate and water

So what if 5% of your NaOH ISN'T NaOH? Then your molar mass is no longer 40 g/mol, it's 38 g/mol. Or maybe you only have 80% of your NaOH, so it would have a molar mass of 32 g/mol. And that's before we even know what that other 5% and 20% ACTUALLY is - if it's all sodium carbonate, then it would be 5.3 g/mol and 21 g/mol, and if it's all sodium oxide, then it would be 3.4 g/mol and 14 g/mol. If you've gotten lost in numbers, that's kinda my point here - when we don't know how much of the NaOH IS ACTUALLY NaOH, we end up becoming lost in numbers

So you see, because we don't know how much of the NaOH has reacted with the environment, it's not a good primary standard
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Dear VCE 3/4 Chemistry students: you do not need to know how to do pH calculations for your exam. That is all.

Chocolatepistachio

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Re: VCE Chemistry Question Thread
« Reply #8682 on: August 12, 2020, 10:55:40 pm »
0
Hello
would this be right

Coolgalbornin03Lo

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Re: VCE Chemistry Question Thread
« Reply #8683 on: August 13, 2020, 08:57:43 am »
0
Iím doing the 2010 exam and I know itís from an old study design but itís asking me to calculate pH something which we learnt in unit 1/2 Chem......will this be necessary to know as it has been taught before?
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keltingmeith

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Re: VCE Chemistry Question Thread
« Reply #8684 on: August 13, 2020, 09:19:43 am »
+6
I’m doing the 2010 exam and I know it’s from an old study design but it’s asking me to calculate pH something which we learnt in unit 1/2 Chem......will this be necessary to know as it has been taught before?

No - it was in the exam then because it was in the old 3/4 study design.

Hello
would this be right

I have no idea why some of those are dots and some of those are Xs - man VCE teaches weird things. Otherwise, all of it's right except for your bond angle for COCl2 - think about it, if you have three things in a circle, and they're equally spaced apart*, what angle would you expect them to make?

*=okay, so the angle between the chloride and oxygen bonds will be slightly different to the angle between the two chlorides, but they'll be approximately the same, and that should be good enough for VCE. See your teacher, since they're the ones that'll be marking this stuff.
« Last Edit: August 13, 2020, 12:34:23 pm by keltingmeith »
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Dear VCE 3/4 Chemistry students: you do not need to know how to do pH calculations for your exam. That is all.