I couldn't find the proper key combination to write the subscript part of it but is this correct?

 

H5C3O

 

would mean Five Hydrogen Atoms, Three Carbon Atoms and One Oxygen?

You would normally write it with the carbon at the start:

 

C3H5O

You would normally write it with the carbon at the start:

 

C3H5O

 

Would it really matter if I wrote it as I did though or is it just good habits?

well obviously we know what you are talking about but it is not actually correct, in that it is the wrong way to right it, its likes writting jardy instead of jordie, similar, we know what your talking about, but its wrong!

 

i think in exams you would have not got the mark if that was your answer.

here, take a peek: http://en.wikipedia.org/wiki/Citric_acid