Molecular formula

[kenhung123]

If we have an alkanol for e.g. CH3OH, is that the molecular formula or semi structural? Like I am wondering when we are asked to find molecular formula does it mean we just group all elements together like CH4O or we must show it so it looks like there is a functional group? Maybe even something like C2H6O vs. C2H5OH

[simonhu81292]

semi structural is like CH3CH2CH2OH,, instead of C3H7OH
and is better to show the functional groups...
however in exams it might be either... so knowing both ways are good
if you were asked, look at their molecular formula if shown ..

[kenhung123]

So if it says find the molecular formula for the following molecule and its ethanol, would C2H6O and C2H5OH be both accepted?

[simonhu81292]

C2H5OH ...
easier for examiners to identify but i suppose both are acceptable...
anyone ... correct me if am wrong ...

[Stroodle]

If it asks for the molecular formula, I'd stick with C2H60, as the OH is showing the structure of part of the molecule.

[Potter]

If it asks for the molecular formula, I'd stick with C2H60, as the OH is showing the structure of part of the molecule.

I'd say leaving it as C2H5OH would be acceptable, as you rarely see ethanol written as C2H6O. Same thing with ethanoic acid. As long as the formula shows how many atoms make up the molecule, it should be fine.

What they don't want is if you had say.. C6H12O6 and you simplified it to CH2O(which is the empirical formula).
I even think a semi structural formula can be considered a molecular formula, though I wouldn't suggest doing it.

[Stroodle]

Yeah I reckon C2H5OH would probably be acceptable too, but at the same time I think C2H60 would definitely be acceptable Each to their own I guess..

Edit* As an example in last years exam it was clear in one of the questions that there was an OH group in a molecule and the question was to state the molecular formula. In the assessment report it says it's C2H4O2 and that "Students should know the differences between molecular and semi-structural formulae".

[kenhung123]

So CH3COOH is considered semi structural? If so I think it is best to leave in form CxHyOz

[Stroodle]

Yeah, CH3COOH is semi-structural