Some questions... (1 Viewer)

[Aerlinn]

1. What are the advantages of high performance liquid chormatography for the analysis of durgs compared to an analysis technique based on column chromatography? Hmm...
2. In gas liquid chromatography, why is the column packed with very fine particles?
3. AAS: A solution of distilled water can give an absorbance reading that was not zero. Suggest a reason for this.
According to my teacher, the answer is that the water may be contaminated and the contaminants might absorb light, but the glass might also absorb. Glass? I thought AAS involved spraying solution into a flame. You don't have something like a beaker holding the solution, do you?
4. What does a flame ionisation detector (gas liquid chromatography do)?
5. Last, quick question-- 2.200g sample of an unknown organic compound was extracted from plant material. When burnt in oxygen, hydrogen in the ocmpound was converted to 1.32g of h20, carbon to 3.23g of CO2 and the remainder of the compound was oxygen. Find the empirical formulao f the compound.
For this, I was wondering, how do we find the number of moles of oxygen? I got CH2O3 as answer, and I figured I'd done something wrong 'cause the answer's CH2O.

 

[xiao1985]

a) HPLC - fast, good result... dis adv: you can only do small amount (up to 1g i think) at a given time

b) not too sure about this one... but i guess it's to let the gas through the column...

c) your teacher is wrong (partially) AAS use a tube to "suck" the solution out of the beaker/volumetric flask... so saying glass can absorb is kinda incorrect... but yes, water maybe containminated with metal

d) sorry can't help you on that one... i have no idea myself...

e) 1.32 / (16 + 1.008x2) = 0.073 moles of H2O = 0.146 moles of H
3.23 / (12 + 16+16) = 0.0734 moels of C
lastly, 0.136 x 1.008 + 0.0734 x 12 = 0.955 g , which belongs to carbon and hydrogen... remaining 1.24 g of O = 0.078 moles of O

hence ratio of CHO is 1:2:1

 

[Aerlinn]

a. A good result, and only able to do a small amount how? Heh, I'm terrible with my stuff on these instrument analysis things. I figured that HPLC was a type of column chromatography... are there any differences/ major differences?
b. Maybe...
c. She would probably have a hissy fit if I told her xD
d. That's ok
e. That makes sense. I did it differently by calculating the no. moles of O the same way as with C and H, getting the no. moles O using the 1.32g and 3.23g masses, and adding the no. moles from each together... Not too sure, but is there anything wrong with that, ie. you get more O than you started with?!

 

[xiao1985]

a) i don't know the detail how would the mechanism differ, but the end result is that HPLC gives you a better separation and faster separation, except you can only do small amounts (~1g each chromotography) comparing to column (up to say, 50g or more)

e) oh i see...

you see, what you calculated is the O's you used from ATMOSPHERE not in the molecule originally (ie, when you burn the stuff, it uses O form atmosphere) which is why you get 3 instead of 1 (CO2 and H2O =3O's)

 

[Aerlinn]

e. Ek *wacks self* It was burnt in oxygen. I can be a little dumb sometimes.