# Oleum concentration description

What is the meaning of "free" $\ce{SO3}$ and "combined" $\ce{SO3}$ in the oleum sample concentration description?

Also what are these maximum and minimum values of % labelling in oleum sample?

• There is virtually no SO3 molecules in oleum, as they are strongly Lewis acidic. – Mithoron Jun 26 '17 at 13:53
• Guys, I don't understand how this question is unclear. The question makes sense. And to consider, the asker can only ask about the terms that they have learnt. – Pritt says Reinstate Monica Jun 26 '17 at 14:39
• sigh I guess this "combined" is SO3 + H2O and free the rest as if there was just H2SO4 and SO3 unreacted which isn't true. – Mithoron Jun 26 '17 at 15:15
• The English wiki (en.wikipedia.org/wiki/Oleum) is not discerning "free" and "combined" $\ce{SO3}$, still working with fuming $\ce{H2SO4}$ with 30% and >60% $\ce{SO3}$ (sigmaaldrich.com/catalog/product/aldrich/…) keeps a mark. – Buttonwood Jun 26 '17 at 16:13
• I also think this should be two separate questions. – pentavalentcarbon Jun 26 '17 at 17:10

I think the term combined "$\ce{SO3}$" is nothing but $\ce{SO3}$ of $\ce{(SO3 + H2O)}$ and free $\ce{SO3}$ means that is $\ce{SO3}$ in oleum sample.