# Convert wt% Mg into mole% MgCO3

Submitted by jimi on Mon, 03/29/2010 - 07:54

Hello everyone!
I was examing high-magnesium calcite skeletons using EDS detektor.

Elem    Wt %  At %
C K  12.71  21.00
O K  46.93  58.21
NaK    0.49  0.42
MgK    2.24  1.83
SrL    0.45  0.10
S K    0.27  0.17
CaK  36.91  18.28
Total  100.00 100.00

How to convert these results into mole% MgCO3?
I would be grateful for your help!
Cheers,
jimi

weight % = mass compound/total mass *100

assume 100 grams of sample

take the first 12.71% of C

that means that you have 12.71 g of C convert to moles
repeat for each element on your list to find the moles of each
add up all the moles to find total moles

then %C by moles becomes      moles of C/total moles *100

repeat for each element to find the mole % of each element.
since MgCO3 has a ratio of Mg:C:O of 1:1:3 use that relationship to find the % of MgCO3 from the mole %

Hi,
Thank you so much.
I converted all elements to moles to find total moles, than I found % moles of each element.
But What should I do next?
How should I use the relationship (1:1:3) to find the % of MgCO3 from the mole % ??

From the data I am assuming that you are analyzing probably for water hardness, since your calcium and oxygen are high.

We can safely assume that you have other carbonates, such as CaCO3, SrCO3 and perhaps some Na2CO3 and NaHCO3.

Assuming we have similar calculations I found the following mole percents.
C = 21.0%
Mg = 1.8%
O = 58.2%

We have to make some assumptions, sources of carbon and oxygen are more than just MgCO3, and the only source of Mg is MgCO3, thus the 1.8% Mg tells us that we should expect the contribution of MgCO3 to be
1.8%Mg, 1.8%C and (1.8*3) = 5.4%O    1:1:3    This was done just to determine the element that we could use for the calculations, we cannot use C nor O, we have to go with Mg

Remember that at first we assumed 100g of sample and when you found the moles of Mg you should have gotten 0.092 mol Mg, that means that your sample has 0.092 mol of MgCO3

convert that to mass and you get 7.756 g of MgCO3 in your sample of 100g, which equals about 7.8%MgCO3

I probably made it a little more complicated than it needed to be but let me know if it does not make sense.

Thank you for your comment but still I do not know how to resolve my problem...
I was analysing part of echinoderm (Echinodermata) skeleton (polished sample) which is composed of high-magnesium calcium carbonate. Geochemical measurements were made using EDS detector (see:
http://en.wikipedia.org/wiki/Energy-dispersive_X-ray_spectroscopy ).
This spectroscope provides only elemental compositions in wt% but I need to know mole% of MgCO3....
I will write you some examples from the literature tommorow where wt% of Mg and mole% of MgCO3 are given... (of course the authors did not mention how did they convert wt% Mg into mole% MgCO3)...
cheers,
jimi

Here are these examples:
Mg wt% and mole% MgCO3
1.5 -> 6.1
2.02 -> 8.2
2.5 -> 10.1
3.11 -> 12.5
....