Methods of Analysis and detection! I need help with this question!

A gas liquid chromatogram of the alcohols found in beer are shown. The alcohols are ethanol, butan-1-ol, methanol and 2-methylbutan-1-ol. The graph is in the attachment. The questions asks: 'Suggest which peak on the chromatogram was formed by each alcohol.' I mean how are you supposed to know?!
(Sorry about the rough drawing)

chromatography.bmp285.0KB

Reply 1

shenzys

OP

17

Oh and can someone please explain the M+2 Aand M+4 peaks and the whole thing about chlorine isotpes and bromine isotopes. How are you supposed to know which peak is for what isotope and what ratio!???

shenzys

A gas liquid chromatogram of the alcohols found in beer are shown. The alcohols are ethanol, butan-1-ol, methanol and 2-methylbutan-1-ol. The graph is in the attachment. The questions asks: 'Suggest which peak on the chromatogram was formed by each alcohol.' I mean how are you supposed to know?!
(Sorry about the rough drawing)

The highest peak has to be ethanol - i't a beverage!

Then there are two peaks that take longer .. and there are two molecules that are heavier than ethanol. The faster trace must be methanol.
Out of the two heavier ones the one with the greater retention time (i.e. slowest) is probably the one with greater intermolecular forces i.e the straight chain butan-1-ol

shenzys

Oh and can someone please explain the M+2 Aand M+4 peaks and the whole thing about chlorine isotpes and bromine isotopes. How are you supposed to know which peak is for what isotope and what ratio!???

CHLORINE
-----------------------------------------------------------------------
Chlorine has two isotopes 35 and 37. A molecule is made of two atoms ( Cl2 )
so the molecules can have a mass of 70, 72 or 74
ie. M, M+2 or M+4

statistically one in every four atoms is 35Cl and three in every four is 37Cl

The probability of BOTH atoms being 35Cl is 3/4 x 3/4 = 9/16
The probablilty of one being 35 and the other 37 is 1/4 x 3/4 = 3/16
The probablity of one being 37 and the next being 35 is 1/4 x 3/4 = 3/16
The probablity of BOTH atoms being 37Cl = 1/4 x 1/4 = 1/16

Therefore you should see traces in the mass spec at intensity ratios of 1:6:9 corresponding to 37Cl37Cl, 35Cl37Cl and 35Cl35Cl respectively

OP

Ohhhh thank you!

OP

Ok, I need help with another question please! Its from a question paper, jan 02. Question 1e (ii). I know the equation is 1/2 base * height, but what units do I use? and what I get from measuring this isn't the same as what's there in the mark scheme.
Both question paper and mark scheme are attached. Please help!

you don't use units - you are asked for a percentage - THAT's the unit

It doesn't matter what you are using to measure, mm, cm inches etc as long as you are consistent.

percentage = amount/total amount x 100

Reply 7

shenzys

OP

17

oh but they use the base*height/2 equation to get the area. bt ther height and base that they use in the mark scheme, I just don't get how they get it. I easured it with my ruler, and it doesn't match the mark scheme?