I am not sure about mchem, but you can try mhchem, just add \require{mhchem}:
\require{mhchem}\ce{C6H5-CHO}\\
\ce{$A$ ->[\ce{+H2O}] $B$}\\
\ce{SO4^2- + Ba^2+ -> BaSO4 v}
yields
[tex]\require{mhchem}\ce{C6H5-CHO}\\
\ce{$A$ ->[\ce{+H2O}] $B$}\\
\ce{SO4^2- + Ba^2+ -> BaSO4 v}[/tex]
For some reason I can't make it work globally
, will try again later.
Edit: OK, I am an idiot, I forgot to add a comma, should work now.However, I prefer equations and formulae formatted just with standard UBBC tags - we already have plenty of these and it is never a good idea to mix different styles of entering the same kind of data in the database. For example it will make automatic tagging of posts (that we may want to implement one day) much more difficult if the formulae were entered sometimes with UBBC tags and sometimes with LaTeX.