[dina]

Can someone explaine to me next:
How is possible that one chemicals (4-methylaminophenol sulfate) with CAS number 55-55-0 have two formulas and different molecular mass? Which of theese  formulas and wich molekular mass is corect?

[sjb]

What are the two formulae you have found?

[dina]

once formula is C14H20N2O6S Mr 344,39g/mol, and the other is C7H9NO*1/2H2SO4 Mr 172,19 g/mol.

[Arkcon]

In the second case, you've found an archaic or jargon term for the molecule.  This happens from time to time.  If you really want to get to the bottom of occurrences like this, identify the reference you're using.  Googleing for your first molecule gives N-Methyl-p-aminophenol sulfate, sometimes called Metol.  You second formula gives an unrelated compound, that doesn't have the common name Metol.

[sjb]

What are the actual structures. I think what you may have found is a salt which in one case has one mole of sulfate in the formula, and the other has one mole of amine.

[dina]

The problem is that different manufacturers on offer have this chemical under the same name Metol but with different formulas and molar masses as stated above. the manufacturer of Metola Mr 172.19 g / mol claims to be the same chemicals because they have the same CAS number and this means that the structural formulas are the same.
Is that correct?

[Arkcon]

claims to be the same chemicals because they have the same CAS number and this means that the structural formulas are the same.
Is that correct?

This part is not correct.  CAS numbers are assigned just by asking.  They are not derived from structure.  A company can ask for a new CAS number for a previously known chemical, just because they feel lie it.

You are right, two different chemicals shouldn't have the same CAS number.  But they can have the same common name.  Googling for 'metol' tells me it is an archaic name for 'methanol', so there are at least three different chemicals, each with different CAS #, a different structure, and a different MM, all under that common name.

[DrCMS]

Has nobody bothered googling this yourselves?  The two structures are both correct and the molecular weight of one is twice the other.  The smaller one is with 1/2 a molecule of sulphate with one aminophenol and the other is two molecules of aminophenol with one sulphate between them.  They are the same thing.

[Arkcon]

Good catch.  Yes, this is also a common thing to do.  Metol appears to be a common name for 4-(methylamino)phenol sulfate.  If it appears as a dimer of the organic as a sulfate salt, then it can be identified as a dimer or as a monomer with "a half" of a sulfate.

[dina]

All you said was clear to me. One Mr represents hemisulfate and the other sulfate.
Which of the two structures is real? The manufacturer offering methanol M = 172.19 g / mol gives the following explanation: "Both molecular weights are right, as they represent the same compound and structure. M = 172.19 stands for 4- (methylamino) phenol molecule with 1/2 a sulfonic acid molecule. However, in reality it is impossible to split a molecule why "in the bottle" the compound forms a molecular weight of 2 * 172.19 = 344.38 ".
Other manufacturers offer metol with Mr = 344.38 g / mol. When preparing molar solutions for these two chemical weighers is different.

[sjb]

What is the definition of mol in this case?

[Arkcon]

Which of the two structures is real?

You might think this is a meaningful question, but really, this is purely philosophical.  Like sjb: said, define mol for us.  At any rate, you can't have half of a sulfate, you have to realize.

[dina]

ok .. i will try again to ask your help  because it is very important to me. If i need to prepare 1M solution and i have only salt called Metol and CAS number 55-55-0. And I find there are two forms of the same chemicals with different molecular weights, which of these two molecular masses should be taken into account, as like hemisulfat or sulfat

[sjb]

...If i need to prepare 1M solution ... which of these two molecular masses should be taken into account

A 1 molar solution with respect to what? The amine, or the sulfate? Do you have an example preparation that you are following and adapting?

[dina]

1M solution of Metol  4-methylaminophenol sulfate