Chemistry 3/4 2013 Thread

[Mr Keshy]

I hate the questions where there are the different dilution factors and different units of concentration like ppm and M. I wanna just plug numbers into formulas not think.

Step right into Physics

[Lasercookie]

I don't know if this is just me, but I don't think I'm enjoying chromatography very much. I really liked the straightforward nature of the other analytical techniques discussed up until this point, but it seems to me that chromatography is very much so a theory-based topic with quite challenging problem-solving questions. Perhaps it's because I'm not learning it well, or because it's a new concept. Does anyone understand me?

It's probably because it's a new concept.

But yeah, chromatography, AAS, NMR, UV, IR and all those other analytical instruments you learn about is fairly theory based and the concepts are hard to get your head around at first.

Chemguide I think has been mentioned a lot here as a good resource, and it covers a lot of this type of material really well. I'd strongly urge you to spend the time reading through the pages it has on instrumental analysis. http://www.chemguide.co.uk/analysismenu.html#top

[Stick]

Thanks! I think I just need to go over high performance liquid chromatography a bit more. I get the others now.

[Homer]

Cereal Solution Conc 6.89? absorbance: 0.410
1. conc 0 absorbance: 0.030
2. conc 2.5 absorbance: 0.158
3. conc 5.0 absorbance: 0.342
4. conc 7.5 absorbance: 0.451
5. conc 10.0 absorbance: 0.595

Concentration was measured in where

[Daenerys Targaryen]

@Homer; teacher said that it was 6.25. that was from suggested answers. I got 6.6655 from a calculator.

[9_7]

If a graph had an absorbance of not zero, what could you say in this case? :s

[jono88]

anyone done the chromatography of a vegetable extract? Using Thin Layer Chromatograpy, why do different solvents produce different chromatograms? and Why must the line where the extract is placed on the plate be above the level of the solvent at the beginning of the experiment? Ty

[Daenerys Targaryen]

If a graph had an absorbance of not zero, what could you say in this case? :s

If it was suppose to have zero absorbance, maybe the thing was contaminated?

[Bad Student]

anyone done the chromatography of a vegetable extract? Using Thin Layer Chromatograpy, why do different solvents produce different chromatograms? and Why must the line where the extract is placed on the plate be above the level of the solvent at the beginning of the experiment? Ty

Different solvents produce different chromatograms because samples are soluble in each solvent to a different degree, therefore, they will travel a greater or lesser distance.

The line where the extract is placed on the plate must be above the level of the solvent at the beginning of the experiment so that the extract doesn't dissolve into the solvent.

[Edward21]

I have a question from VCAA 2007, "Which one of the following is least likely to be a product of a redox reaction between sulfuric acid and zinc metal? A. H2, B. H2S, C. SO2, D. SO3 ....the answer's D, I just don't know how the hell to get there!? 

[sin0001]

You know that sulfuric acid is gonna act as an oxidant when reacted with Zinc metal, therefore the elements in it (H,S,O) can only reduce in oxidation no.
Sulfur's ox. no. should be lower than +6 & Hydrogen's should be lower than +1, after this reaction (oxygen stays as -2). And since 'D' is the only option where the oxidation number of hydrogen/sulfur has not decreased, this is the answer

[Bad Student]

I'm very angry at question 1d on page 41 of the AN chem guide. My answer was 2-chloropent-3-ene. but the definitive Thushan answer was 4-chloropent-2-ene.

I thought that you were supposed to number the functional groups so that their sum was the lowest possible number. Why is the double bond deemed the principle functional group and given the right to dictate the number of the chloro group when assigning the chloro group to carbon-2 would give a lower sum? It seems a bit racist to the chloro functional group to give priority to the double bond.

[lzxnl]

I think what Thushan is trying to get to is this. Pent-2-ene is the name of the hydrocarbon skeleton. Pentene by itself is insufficient as we do not know where the double bond is.
It explains why we would name 5,5,5-trichloropentanoic acid that, even though calling it 1,1,1,trichloropentan-6-oic acid produces a lower sum.
The rule is, you have to minimise the number for the parent functional group. In the first case, it is the double bond. In the second case, it is the carboxylic acid. Read http://en.wikipedia.org/wiki/IUPAC_nomenclature_of_organic_chemistry if you're still not sure, especially the bit on numbering.
I admit, this system is confusing, but we need rules somewhere and that is how they have been set.

[Bad Student]

Thanks nliu1995. I didn't know about the hierarchy of importance for functional groups in nomenclature.

[Limista]

Does NaOH react with water and carbon dioxide in the air to form carbonic acid?