Analytical Chemistry Questions

[LJP07]

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!!

[JMJones0424]

The first question is relatively easy. I won't answer for you, but it should be easy to figure out if you remember that an acid and a base combine to produce water and in this case a percipitant, which is obviously hydrated.

 

On the other two questions, I'm clueless. It's been too long since chem class for me, sorry.

[LJP07]

Well,

 

Mg(OH)2 + H2SO4 --> MgSO4 + 2H2O

 

I'm just unsure on the losing weight parts really...

[JMJones0424]

You are creating magnesium sulfate in an aqueous solution. It is "hydrated", and the progressive heating of the percipitant drives out the water. Using the final weight as pure MgSO4, you should be able to write a stochiometric equation for each of the substances.

[LJP07]

As much as I understand your logic, I'm still completely lost everytime I read the question :phones:.