Analytical Chemistry Questions

sizabletomato

Hi, just wondering could people here give me a hand on some of these. I've tried some of them and want to see if I'm right, and then there's one or two which I haven't got a clue how to answer, need some direction with. Thanks for any responses.

Q. Magnesium hydroxide and sulphuric acid react in water to give compound A. TG analysis of A was undertaken.
Gentle heating at approximately 100 ºC results in 2 mass losses. The remaining mass after the first event is 56.1% and after the second event it is 48.8%. A third event, after extreme heating in excess of 300 ºC, is seen, after which the mass remaining is 16.40 %. Identify A, and write a balanced equation for its formation. Write balanced equations for all three events observed in the TG experiment.
[Atomic masses: H = 1.01, O = 16.00, Mg = 24.31 and S = 32.06]

Haven't the foggiest how to do the above, tried googling for help and still unsure.

Q. Injected solvent passes through a chromatographic column in 2.0min while two drug substances require 6.0 and 10.0min respectively. Calculate the capacity factors k' for the two solutes. What is the selectivity of the separation?

k'A = t R - tM / tM <-- I think this equation is used but am unsure...tm/tm do you just put 6 in for both top and bottom?

and I think this equation is used for the selectivity;

a = k 'B / k 'A but can't answer that until I do the top half which I'm sort of unsure of as well.


Q. A chromatogram shows an eluted solute peak, with ideal Gaussian shape at Tr = 10.0min and w1/2= 1.0min.

(i) How many theoretical plates are present?
(ii) Find the plate height if the column is 10cm long?

This is my attempt at that...

N = 5.54[tr/ w1/2]^2
5.54[(10min)(1min)]
55.4 theoretical plates present

HETP = L/N... 100mm/55.2 ==> 1.81

So can anyone help with these three questions and correct my if I said or made an error above.

Thanks!!

Kevster

I've just looked at the first question, but you should be able to solve it by first calculating/adding-up the atomic masses of Magnesium Hydroxide and Sulphuric Acid. Then calculate the exact mass lost after each of the three events, and infer the chemical equations thereafter.

I'm not sure how the temperture should be brought into it, but maybe one of the elements involved isn't reactive at 100C but is at 300C...? If this is true, then it wouldn't be involved in the first two events.

Kevin

Edit: I've just looked at your other questions and am appalled Dude, listen more in class and take better notes. If you're unsure about such things, then ask your lecturer. They're there to help you when you don't know how to answer something.

craggles

I recommend the book entitled Quantitative Chemical Analysis. It's ****ing whopper for chromatography etc. Get it in the library.

sizabletomato

I'm using Quantitative Chemical analysis. I would just like confirmation if I'm right or wrong.
Kevster, I emailed the lecturer, but haven't heard a word in a week.
I know how to do the second question, it's just the last one. I've an exam on this soon and would like to know if I'm right to I'd appreciate help please.

Was my attempt right or wrong for the last question i.e.;

Q. A chromatogram shows an eluted solute peak, with ideal Gaussian shape at Tr = 10.0min and w1/2= 1.0min.

(i) How many theoretical plates are present?
(ii) Find the plate height if the column is 10cm long?

This is my attempt at that...

N = 5.54[tr/ w1/2]^2
5.54[(10min)(1min)]
55.4 theoretical plates present

HETP = L/N... 100mm/55.2 ==> 1.81mm

(or do I have to convert minutes to seconds or what's the story)

Thanks for help.

Metalpanic

Hi sizabletomato,

This site is very good for chromatography equations and has a calculator to give you answers if you want to compare with your own.

http://mtc-usa.com/calculators_chrom.asp#

N = 554 and HETP is 0.181 mm I think!

sizabletomato

Thanks for that link...good resource!

SOL

well the first thing is that you have MgSO4.xH2O so you will have loss of water as a mass loss event, the final compound will be chemical decomposition to give you a more simple Mg compound with mw 16% where 48% is mgso4.


right?