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March 29, 2024, 12:40:35 am

Author Topic: VCE Chemistry Question Thread  (Read 2313157 times)  Share 

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Blondie21

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Re: VCE Chemistry Question Thread
« Reply #285 on: February 03, 2014, 10:20:39 pm »
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Thanks :) what would the reduction half equation be?
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lzxnl

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Re: VCE Chemistry Question Thread
« Reply #286 on: February 04, 2014, 02:27:39 pm »
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Thanks :) what would the reduction half equation be?

Somehow I managed to misread oxidation as reduction. Sigh. Holiday mode does stuff to your mind.

I gave you the reduction equation above, where iron ions are reduced to iron metal.
The oxidation equation would be simply Al(s) => Al3+(s) + 3e-

There we go. I finally demonstrate that I know what oxidation and reduction are -.-
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ealam2

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Re: VCE Chemistry Question Thread
« Reply #287 on: February 04, 2014, 03:36:41 pm »
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Hi! Could someone please explain HPLC (High Performance Liquid Chromatography) in detail? Having an excursion tomorrow and will be doing a prac on this concept. Thanks!  :)

Edward21

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Re: VCE Chemistry Question Thread
« Reply #288 on: February 04, 2014, 07:18:56 pm »
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Hi! Could someone please explain HPLC (High Performance Liquid Chromatography) in detail? Having an excursion tomorrow and will be doing a prac on this concept. Thanks!  :)
This is a form of column chromatography, that involves separation of compounds (as in all types of chromatography) but generally we're dealing with compounds whose molar masses are > 300gmol that cannot be vaporised, if they could be then we'd use Gas Chromatography. Our stationary phase is heaps of these tiny little balls in the column (silica), we have small sized balls to increase the surface area, to ensure maximal resolution (ie. better separation) of our sample. The mobile phase is a non-polar solvent (often hexane) which is passed through the column at High Pressure *hint hint* because of all these small little balls with so much surface area, we need high pressure to be applied to our mobile phase to move the compounds through :) the rest is essentially the same as basic chromatography principles, the higher the temp. of the column the faster the compounds will be eluted, the more soluble a component is in the mobile phase, the quicker it will move through the column just like in Gas-Column Chromatography. Chemguide.co.uk would have a more detailed explanation :)
« Last Edit: February 04, 2014, 10:52:21 pm by Edward21 »
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scribble

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Re: VCE Chemistry Question Thread
« Reply #289 on: February 04, 2014, 09:36:13 pm »
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o.O i always thought that hplc used a solvent as the mobile phase, not a gas.

ealam2

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Re: VCE Chemistry Question Thread
« Reply #290 on: February 04, 2014, 09:45:40 pm »
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o.O i always thought that hplc used a solvent as the mobile phase, not a gas.

Yeah I read that in chemguide, unless gas is a solvent? Is it? Sorry I feel so dumb right now.  :-[

scribble

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Re: VCE Chemistry Question Thread
« Reply #291 on: February 04, 2014, 10:03:12 pm »
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:S ive been under the impression that with hplc, the stationary phase was the packing inside of the column (eg silica) and then the solvent was the mobile phase. Then there's a pump attached to the front of the column that constantly pumps more of the solvent/mobile phase through the column, sweeping the sample along and with it and separating out the components.
in normal phase chromatography, the stationary phase is more polar than the mobile phase and so components that are highly polar have a higher affinity to the stationary phase than a component that is less polar. hence, polar components spend more time adsorbed onto the stationary phase and so would stay in the column for longer/have a higher retention time than non polar components. in reverse phase chromatography, the stationary phase is less polar than the mobile phase so the opposite occurs; polar components are eluted from the column faster than non polar components and have a lower retention time.



EFPBH, you pretty much need to work out the mole for any chemistry question. to find the empirical formula, you're trying to find the atom ratio of each element in the hydrocarbon. moles are just a convenient way of counting atoms.

ealam2

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Re: VCE Chemistry Question Thread
« Reply #292 on: February 04, 2014, 10:09:12 pm »
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Thank you scribble and Edward21!  :) Hope I understand it more after my excursion and that it all sinks into my brain.   :)

EFPBH

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Re: VCE Chemistry Question Thread
« Reply #293 on: February 04, 2014, 10:22:19 pm »
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Permethrin, C21H20Cl2O3, is one of the pyrethroid insecticides that are replacing organochlorine and organophosphate compunds. if a sample of permethrin contains 0.480g of carbon then the mass of chlorine present in the sample is ?

could you please explain each step, much appreciated :)

scribble

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Re: VCE Chemistry Question Thread
« Reply #294 on: February 04, 2014, 10:28:22 pm »
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work out how much carbon you have in mol. then, as you know that the carbon, chlorine ratio is 21:2, you can work out how much chlorine you have in mol. then you can convert that to mass.
n(C)=0.480/12.0 = 0.0400mol
n(Cl)=2/21 * n(C) = 2/21*0.0400=0.0038095mol
m(Cl)=nM=0.0038095*35.5=0.135g to 3sigfigs

Edward21

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Re: VCE Chemistry Question Thread
« Reply #295 on: February 04, 2014, 10:55:13 pm »
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Yeah I read that in chemguide, unless gas is a solvent? Is it? Sorry I feel so dumb right now.  :-[
I fixed it up now, my mistake!
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ealam2

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Re: VCE Chemistry Question Thread
« Reply #296 on: February 05, 2014, 03:58:06 pm »
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Thank you very much, Edward21! :)

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Re: VCE Chemistry Question Thread
« Reply #297 on: February 05, 2014, 06:30:04 pm »
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There is a mixture of aluminium powder and iron (III) oxide.
When it is ignited, a redox reaction occurs to form molten iron.

How would we write the overall equation for this process?

Cheers!

lzxnl

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Re: VCE Chemistry Question Thread
« Reply #298 on: February 05, 2014, 10:07:00 pm »
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You'll find the answer is on this/last page. Including a very stupid mistake by me in misreading oxidation as reduction.
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Blondie21

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Re: VCE Chemistry Question Thread
« Reply #299 on: February 06, 2014, 05:28:01 am »
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Well...given the question...I'd say it would be Fe2O3(s) => 2Fe(l) + 3O2- (s) + 6e-

Edit: I've read the question incorrectly; this is the reduction half equation.

I asked my teacher about this and he said that the 3O2 isn't supposed to be there and now I'm pretty confused. I was wondering if someone could show me step by step to find this half equation.

Btw, if this is a reduction half equation, why aren't the electrons on the left side?

Thanks :-)
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