Water's Noobie Chem Thread.

[Water]

Name one or more analytical technique that could be used to:

C) Verify the identify of a sample of an organic compound.

So I said NMR, HPLC, MASS, Infrared.

Why can't I use HPLC though? Solved



D) Identify the components of a mixture of alkanes.

Book said only GC. But, why can't I used HPLC? Solved


14B) Determine the concentration of a silver nitrate solution

The book said volumetric, AAS , Gravimetric

But why can't I use UV Vis? Solved

[vea]

14B) UV Visible is only for coloured or organic compounds.

[Water]

14B) UV Visible is only for coloured or organic compounds.

vea, the book said

UV Vis can be used for identifying the presence of metal ions; even if the metal ion itself is not colored, it may be possible to analyse it by converting it into a colored compound.


Also UV Vis occurs for such electron transitions of atoms, ions or molecules

>;


Could the book be wrong for this answer? For question 14B that is.

[azngirl456]

Some alkanes are volatile such as methane and ethane. So it's best to use GLC so you can vaporize the whole mixture to identify all the components whereas using HPLC, you might lose some of the components through evaporation

[Greatness]

Hmm i thought vea's answer was pretty accurate... Thats what we learnt in class.

[Water]

Azngirl, in HLPC, isn't the sample, liquid throughout the process?


 

[azngirl456]

Azngirl, in HLPC, isn't the sample, liquid throughout the process?


 

Yes that's correct, which means that you can't use a gas in HPLC. You can use liquid alkanes as the sample for HPLC, but not gas ones.

Because it just says mixture, you're not sure of the states, so I would just go with GLC...because it's possible to make a liquid into a gas

[Water]

Azngirl, in HLPC, isn't the sample, liquid throughout the process?


 

Yes that's correct, which means that you can't use a gas in HPLC. You can use liquid alkanes as the sample for HPLC, but not gas ones.

Because it just says mixture, you're not sure of the states, so I would just go with GLC...because it's possible to make a liquid into a gas

Not sure about the UV Vis one though, we analyzed K2MNO4 with colorimetry, so how is it considered for only organic chemistry as well? ;O

[luken93]

14B) UV Visible is only for coloured or organic compounds.

vea, the book said

UV Vis can be used for identifying the presence of metal ions; even if the metal ion itself is not colored, it may be possible to analyse it by converting it into a colored compound.


Also UV Vis occurs for such electron transitions of atoms, ions or molecules

>;


Could the book be wrong for this answer? For question 14B that is.

I think you may be reading it wrong Water...

"it may be possible to analyse it by converting it into a colored compound." - Silver Nitrate isn't coloured, however you could analyse if Silver was present and/or it's concentration by making it into a coloured solution (can't think of any). But for Silver Nitrate specifically, it isn't coloured, so you can't analyse SILVER NITRATE with UV Vis...

[Water]

But, if that is the case, for volumetric analyst, you would need to use indicator.

And for Silver Nitrate, you would need to precipitate it.


ARGH, this is so confusing. Heineman Book page 120.


UV VIS : Typical Analyte : Low Molecular mass organic molecules

            Typical Sample: Liquid and Gas Samples


But on page 85: UV VIS has wide range of applications for virtually everything.



So Frustrating! *slits wrist*

[luken93]

Volumetric: Add another substance, find end point, determine concentration
AAS: Will give you the concentration of metal (Ag) ions by putting it through the AAS instruments
Gravimetric: Add a substance that will form an Ag precip, from there you can determine the concentration

In UV however, you actually have to change the substance. In the others, another substance is added to allow it to become equi-molar, or to precipitate, but in UV you can't actually use AgNO3 if you get what I mean?

It makes sense to me saying it like that, ultimately you could determine the concentration by creating a coloured compound with the Ag in it, but you can't actually determine the concentration without creating a different substance?

I think I'm going around in circles, and as you say you also need a substance for Grav and Vol as well...

[Water]

luken93, I went to my maths tutor today, he's a chemical engineer  and he said you can find the concentration of AgNO3 via UV Vis ! No Reason why should not be able to, as I suspected  :tickedoff:

[luken93]

luken93, I went to my maths tutor today, he's a chemical engineer  and he said you can find the concentration of AgNO3 via UV Vis ! No Reason why should not be able to, as I suspected  :tickedoff:

Hahaha well then the book is indeed wrong, I was just trying to justify the book, I'm sorry

[zibb3r]

My book agrees with luken93, but my tsfx notes agree with water.....   :buck2: wth?

[Mao]

In UV however, you actually have to change the substance. In the others, another substance is added to allow it to become equi-molar, or to precipitate, but in UV you can't actually use AgNO3 if you get what I mean?

Yes, there are many dye bases to which we can react with the analyte selectively to find their concentration with photospectrometry. In the typical case, we are probably looking at a dye that will react with NO3- only (We add the dye base in excess, and all the nitrate will form a coloured dye). It is correct that we cannot measure some things directly.