Sciencemadness Discussion Board

Question on Iodine molar mass in equations

Utopio - 10-5-2018 at 15:58

Hello! I have become very confused on this problem on ethyl iodide synthesis containing elemental solid iodine. When I was calculating my yield I came to a confusing block. Should I use 127 as the molar mass or 253.8? since it is diatomic. I searched up on youtube, I found nildred and chemplayer in their videos for ethyl and methyl iodide they use 127 as the molar mass, so my question is, why is that?

chemplayer... - 10-5-2018 at 16:23

If you write the reaction out and and work out your stoichiometry using I then the molar mass is 127g/mol. If you use I2 then the molar mass is 254g/mol.

It actually makes no difference.


edit: 254g for I2, 127g for I

[Edited on 11-5-2018 by chemplayer...]

Vosoryx - 10-5-2018 at 16:27

EDIT: Chemplayer not only understood the question better but gave a better answer. I should have waited to reply.

127 is the MW of a single iodine, or iodide, and elemental iodine exists as a diatomic covalent bond, I2, with a mass double that. I don't think that NR put his calculations on screen, so I can't tell what he used as his calculations.
In one molecule of methyl iodide, the iodine only counts for 1/2 of the original iodine molecule. There is only one iodine atom in each molecule of methyl chloride, so each diatomic iodine could make 2 molecules of MeI.
I don't see where he used the MW, unless it was in the audio. (Can't listen to audio RN.)
Does this help? I'm not entirely sure I get the question.
Also, for future reference, this belongs in beginnings.

[Edited on 11-5-2018 by Vosoryx]

Utopio - 10-5-2018 at 16:29

Oh hi chem player! Big fan. I was just recording my yield so if I used 126 then my yield is 44% but if I used I2 then my yield is 88% so Im still confused idk if this is just a nasty brain fog.. Edit omg I'm an idiot I just didn't convert it using the ratio facepalm

[Edited on 11-5-2018 by Utopio]

Utopio - 10-5-2018 at 16:31

Oh thats more descriptive thank you vosoryx

BromicAcid - 10-5-2018 at 16:42

The molecular weight of iodine is 126.90 g/mol. If you have I2 then you're counting two iodine molecules which would have a molecular weight of double that of a single iodine molecule (253.8 g/mol). All of the masses on the periodic table are for the monatomic versions of the molecules. The MW listed for sulfur is listed as 32.06 g/mol even though sulfur is usually found as S8, phosphorus as 30.97 even though it's P4 or some sort of polymer. To avoid confusion it's always assuming monatomic.

When it comes to your reaction, keep this in mind, as long as you're using the correct mass though and you have a balanced equation it does not matter.

E.g.

Na + I ---> NaI (use 126.90 g/mol)
OR
2Na + I2 ----> 2NaI (use 253.8 g/mol)


[Edited on 5/11/2018 by BromicAcid]

LearnedAmateur - 11-5-2018 at 01:31

Quote: Originally posted by Vosoryx  
There is only one iodine atom in each molecule of methyl chloride


I don’t think there are any iodine atoms in MeCl :P

DraconicAcid - 11-5-2018 at 05:24

Quote: Originally posted by BromicAcid  
The molecular weight of iodine is 126.90 g/mol. If you have I2 then you're counting two iodine molecules which would have a molecular weight of double that of a single iodine molecule (253.8 g/mol). All of the masses on the periodic table are for the monatomic versions of the molecules. The MW listed for sulfur is listed as 32.06 g/mol even though sulfur is usually found as S8, phosphorus as 30.97 even though it's P4 or some sort of polymer.


No. The atomic weight of iodine is 126.9 amu. The molecular weight of iodine is 253.8 amu. The molar mass of iodine molecules is 253.8 g/mol. The molar mass of iodine atoms is 126.9 g/mol.

The periodic table lists atomic masses.

Texium - 11-5-2018 at 06:00

Consider this: 100 grams of I and 100 grams of I2 both contain the same number of iodine atoms. In the latter scenario they just come in pairs. So if your stoichiometry is set up correctly, you can use either the atomic mass or the molecular mass to calculate your yield, but if you use the molecular mass, you'll end up having to divide by 2 an extra time on account of 2 MeI molecules being formed for each I2 molecule consumed.

Thus I prefer to just pretend that it's monoatomic for the sake of stoichiometry. It makes things simpler. If you treat it that way, then your yield is simply the ratio of moles of MeI obtained over moles of iodine atoms that you used.

Sulaiman - 11-5-2018 at 06:12

I have noticed that in Wikipedia articles on elements,
many of the elements no longer have a standard atomic weight,
but now have a list of isotope abundances,
because for some elements (Iodine not being one of them I think) the atomic weight depends upon the mix of isotopes,
which can be different depending upon the geographical source.

Just another constant that turns out to be a variable :P
(not that it affects results at my level of precision)

[Edited on 11-5-2018 by Sulaiman]