\definefloat   [chemicalformula] [chemicalformulae]
\setuplabeltext[chemicalformula=Reaction~,chemicalformulae=Reactions]
\setupfloat    [chemicalformula] [location=here,frame=on,inner=\hsize.8\textwidth\dontleavehmode,align={flushleft,lohi}]
\setupcaption  [chemicalformula] [location=right,distance=0cm,width=.2\textwidth,align=flushright]

\setupbodyfont [dejavu,10pt]

\showframe

\starttext

\startplacechemicalformula
    \dontleavehmode\startchemicalformula
        \chemical{2H_2}
        \chemical{PLUS}
        \chemical{O_2}
        \chemical{GIVES}
        \chemical{2H_2O}
    \stopchemicalformula
\stopplacechemicalformula

\stoptext
