Unit 3 Questions MEGATHREAD :)

[Andiio]

would this be a good thing to remember, as in does it make sense? or not..
more titre= lower concentration of unknown
less titre= more concentration of unknown

Not necessarily, as the unknown can be in the conical flask or the burette.

Is transmittance the amount of light/radiation absorbed or not absorbed?

Transmittance is the opposite of absorbed, if you think about it this way, if the atoms absorb 20%, they will transmit 80% of the light.

KBT said Monosaccharides can be used in GC, whilst STAV says otherwise and it will decompose in GC.

Which one is speaking the truth? I don't like some of KBT answers though.

I would go with HPLC, as they generally decompose. However, I'm highly doubtful that VCAA will ever ask this. It's usually AAS vs UV-Vis vs IR vs GC/HPLC, never HPLC vs GC

agreed, an HPLC vs GC question is, in most cases beyond the scope of our course.
I'm really paranoid about some weird as theory questions haha..

[Andiio]

Is transmittance the amount of light/radiation absorbed or not absorbed?

Transmittance is the opposite of absorbed, if you think about it this way, if the atoms absorb 20%, they will transmit 80% of the light.

Thank You; Do we also need to know the ranges of colorimetry and so on? For example 400-750nm etc?

Nah you don't have to know the complementary colour wavelengths etc

I've never seen any Q's asking about transmittance and all :\

[Andiio]

With transesterification, if we were to write the combined eqn,

can we write OH- as the catalyst as opposed to KOH/NaOH? Also do we have to specify that the OH-/KOH/NaOH is (aq)?

Thanks!

Also, when writing the 2nd part of the transesterification, do you use 1 fatty acid to make the methyl ester or are you supposed to use 3 to make 3x methyl ester? :|

Are we meant to write transesterification in 2 separate eqns or 1 combined eqn?

[Sakigami]

chloroalkanes , fluoroalkanes, etc do they all form hydrogen bonds with one another and only with water? what about the dispersion forces since they're hydrocarbons?   

I didn't think chloroalkanes and fluoroalkanes can form hydrogen bonds with each other as there isn't any H bonded to F,O,N for them to form hydrogen bonds with. I believe that since they are polar, they would form dipole-dipole bonds if anything? If a chloroalkane or fluoroalkane has a long non-polar end, I suppose the net dispersion forces will be the strongest force of attraction between molecules.

But yes they do form hydrogen bonds with water as the H's are bonded to an O, allowing for hydrogen bonding to occur.

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Another question I have is in relation to calibration curves;
1 method of doing these questions is to use "dilation factors" to calculate the concentration in the sample. But I find its more logically to calculate the mass in the diluted sample and work backwards from here. Is this an acceptable method of working for these type of questions?

I think you mean dilution factor? hahah
Well yeah i just ratios so much easier - im assuming this is what you do? so you have to fractions and you gotta solve for x?..

Yeah, I do ratios for some of those questions because it makes more sense to me.
What I meant was for instance, you determine c(Pb) in 10ml is = 20.0mg/L
So m(Pb) in 10ml is = 0.2mg
It's a 10ml aliquot from the 100ml flask it originally come from;
m(Pb) in 100ml = 2.0mg
And you're asked to find the concentration of Pb in a 2.0g sample.
% Pb = 2.0 * 10^3 g / 2.0g * 100 = 0.1%

Rather than doing that messy dilution factor some of these practice exams go on about. :S

[luken93]

So, when do we add catalysts?

As much as we can orr? It's just that it's hardly ever mentioned/shown in the assessors report? It's only ever shown if the question actually asks "what is the common reagent catalysts etc etc"

[Sakigami]

So, when do we add catalysts?

As much as we can orr? It's just that it's hardly ever mentioned/shown in the assessors report? It's only ever shown if the question actually asks "what is the common reagent catalysts etc etc"

Yeah, I noticed this too in the practice exams and VCAA exams that they don't seem to mention the catalysts for answers where they haven't specifically asked for them. Personally; if you know them very well, I would suggest you write them down just in case. VCAA may choose to be picking about catalysts this year, so you really don't want to give them reason to deduct marks. A similar case was with the naming of that amine in a past year exam where it wasn't a problem in the past, but they decided to be very picky about it from that year onwards. So I think its good safety measure to include your catalysts.

Having said that though; if you write the wrong catalyst, that's probably more reason for them to deduct marks so I think its a matter of personal choice.

[GuessWho]

In Maltose how many ether links are there? Should we be counting the ether link that's present inside a glucose molecule itself?

[david10d]

It's only 1 ether link. You only count the glucose-glucose link.

[GuessWho]

That's what I thought but in TSSM 2010 they count the ether links present in the individual glucose molecules

[vea]

Technically the part inside glucose is still -O- but it wasn't formed as a result of 2 OH's bonding. I'd say VCAA would accept both? :S

[homosapiens]

Last minute question: but-2-ene is subjected to high temperature and pressure, what will it become?

[Mao]

Last minute question: but-2-ene is subjected to high temperature and pressure, what will it become?

Probably polymerizes.

[Lols123]

what are you guys doing atm?
one more practice paper? looking over notes?

[luken93]

what are you guys doing atm?
one more practice paper? looking over notes?

Watching telly

[Lols123]

would VCAA ever ask us predict how downfield a chemical shift is judging from a structure?
does it work like this: the closer the H or C atom is to an electronegative atom like O, the more downfield it is?