Swarley's Chem Questions :)

[Greatness]

Chrom/Spec tech questions:

1) Do we need to know the difference of normal phase and reverse phase chromatography? Or are tsfx just being ghey?
2) Has anyone encountered a question that requires you to calculate the area of a peak so A=1/2*b *h? Hence we can find the % of a certain substance
3) Shuold we know thee relationships between wavelength, frequency and energy
4) How does a monochromator acutally split the light up so that you only get the desired type of light or whatever?
5) 'In AAS to determine the absorbance of the metal, the detector measures that amount of light before the substance absorbs it; and then it measures the amount when the substance absorbs the light. Thus the absorbance is the difference of the 2 values.' Am i even close? lol

[scocliffe09]

Chrom/Spec tech questions:

1) Do we need to know the difference of normal phase and reverse phase chromatography? Or are tsfx just being ghey?
It can't hurt to be aware of RP. Reverse phase was on the 09 exam, and is legit course material. The main thing is to understand the implications of reverse phase i.e. molecules are eluted in the opposite order.
2) Has anyone encountered a question that requires you to calculate the area of a peak so A=1/2*b *h? Hence we can find the % of a certain substance
I don't remember doing any questions like this but I spose it's legitimate again.
3) Shuold we know thee relationships between wavelength, frequency and energy
Yes, it's basic physics and important to differentiating between NMR, IR and UV/vis/AAS. long wavelength = low frequency = low energy. short wavelength = high frequency = high energy
4) How does a monochromator acutally split the light up so that you only get the desired type of light or whatever?
often a prism is used to split light into a 'rainbow' i.e, to spread light out, then a slit is used to select a specific wavelength. otherwise a machine can be used to filter the light so only a single wavelength can get through.
5) 'In AAS to determine the absorbance of the metal, the detector measures that amount of light before the substance absorbs it; and then it measures the amount when the substance absorbs the light. Thus the absorbance is the difference of the 2 values.' Am i even close? lol
Yep pretty much - I would just change the "measures the amount when the substance" to " measures the amount of light leftover after the substance has absorbed some of the light"

[luken93]

1) Pretty sure we aren't. Came across a Q in a prac exam with it, but it explained how it works enough to answer the question...
2) Nope. Peaks are usually too steep, so peak height is used (from what I've seen)
3) Pretty sure we don't, although it's probably handy to know what wavenumber is
4) I'll wait for someone else to answer this question properly...
5) Yeah that's pretty much correct, they'll first run a test sample that is *pure* so as to erase any indifferences caused by the atmosphere and so on, and then this will be calibrated to 0% absorbance. Then, when the actual sample is run, it will be comparable to this 0% calibration...

[Greatness]

Sweeeet! See Luke ive been studying for chem

[luken93]

Sweeeet! See Luke ive been studying for chem

Good good, I'll pester you tomorrow.

[Aurelian]

4) How does a monochromator acutally split the light up so that you only get the desired type of light or whatever?
5) 'In AAS to determine the absorbance of the metal, the detector measures that amount of light before the substance absorbs it; and then it measures the amount when the substance absorbs the light. Thus the absorbance is the difference of the 2 values.' Am i even close? lol

4) There are a number of techniques. The most common one I've seen in questions uses the ol' prism for dispersion; you physically just split up all the light into its component wavelengths. Other things I've seen are fancy mirrors, which from what I can tell in essence do the same thing. I can't imagine we need to know details though.

5) If we're going to be technical, that's not quite correct. Absorbance is the negative log of the transmittance, the transmittance being the ratio of the intensity of the light after over the intensity of the light before. Thus, absorbance isn't merely the "difference" between the two values =)

[Greatness]

Thanks Aurelian! Yeah i know we dont need to know the finer details, but personally i find that by knowing how something works it help me understand/remember stuff more easily.

[Zien]

Also, since the interference from the flame is constant and reproducible, we can eliminate it by 'chopping' the light into pulses. The pulses of light are transformed into alternating current, detected and recorded, whilst the flame interference is transformed into direct current and not recorded.

^Might want a second opinion on that though; I'm still quite ambiguous over all this extra detail. :p

[Greatness]

Hmm i read about 'chopping' the light,  but yeah we probably dont need to go into this much detail

Another question: If we have to find mols, masses etc of something that has a really big molecular formula, can we just write something like 'let NiC8H14O4 or whatever = x'? So then yuo can just write: n(x), m(x), C(x)  etc.

[Andiio]

Hmm i read about 'chopping' the light,  but yeah we probably dont need to go into this much detail

Another question: If we have to find mols, masses etc of something that has a really big molecular formula, can we just write something like 'let NiC8H14O4 or whatever = x'? So then yuo can just write: n(x), m(x), C(x)  etc.

Where'd you read about 'chopping' the light up into pulses? :O

To answer your question, I'd assume so, as long as its legible? It's a risk though.

[Greatness]

It's in some tsfx notes that ive got. It essentially says that there will be transmitted and emited light, so to ensure that only the transmitted light is detected they chop the light into pulses. This allows the detector to differentiate from the 2 sources.
Ive just summarised that from my interpretation so it may not be 100% correct.

[luken93]

It's in some tsfx notes that ive got. It essentially says that there will be transmitted and emited light, so to ensure that only the transmitted light is detected they chop the light into pulses. This allows the detector to differentiate from the 2 sources.
Ive just summarised that from my interpretation so it may not be 100% correct.

Also, (for UV), they have a mirror-like prism that deflects the beam into a sample cell and the cell in question, which will instantaneously measure the interference present...

[jbebbo]

It's in some tsfx notes that ive got. It essentially says that there will be transmitted and emited light, so to ensure that only the transmitted light is detected they chop the light into pulses. This allows the detector to differentiate from the 2 sources.
Ive just summarised that from my interpretation so it may not be 100% correct.

I think that's correct; the light is pulsed so that the emitted light doesn't interfere with the detector (or so i was told )

[Greatness]

in the reaction: ethene + HCl -> 1-chloroethane, what state is the HCl in?

[Water]

Aqueous, as always.