Displaying a Carbonate Ion with mhchem

Question

I want to write out the formula for the aqueous carbonate ion using the mhchem package. If I write out CO3^{2-}, it looks the way it should be. (See image.)

However, I want to write out: CO3^{2-}_{(aq)}. The 2- charge gets shifted along to the right of the (aq) state symbol. (See image.)

Is there any means of fixing this?

Answer 1

\documentclass[a4paper,12pt]{article}
\usepackage{mhchem}

\begin{document}
 \ce{CO3^{2-}_{\rlap{(aq)}}\; ->}
\end{document}

\rlap suppresses the width of the content, so you need a \; to get the correct spacing.

Answer 2

You can use {}_{(aq)}

\documentclass{article}
\usepackage[version=3]{mhchem}
\begin{document}
  \noindent IUPAC recommendation:\\
  \ce{2H+(aq) + CO3^{2-}(aq) -> CO2(g) + H2O(l)}

  \bigskip

  \noindent Subscript:\\
  \ce{2H+{}_{(aq)} + CO3^{2-}{}_{(aq)} -> CO2{}_{(g)} + H2O{}_{(l)}}
\end{document}

Answer 3

If you don't mind switching packages: the chemformula package in combination with the chemmacros package allows for a variety of different outputs:

\documentclass{article}
\usepackage{chemmacros}

\begin{document}

IUPAC recommendation:\par
\ch{2 H+ \aq{} + CO3^2- \aq -> CO2 \gas{} + H2O \lqd}

\bigskip

\chemsetup{phases/pos=sub}
subscript phase descriptors after the compounds:\par
\ch{2 H+ \aq{} + CO3^2- \aq -> CO2 \gas{} + H2O \lqd}

\bigskip

phase descriptors as subscripts to the compounds directly:\par
\ch{2 H^+_{(aq)} + CO3^{2-}_{(aq)} -> CO2_{(g)} + H2O_{(l)}}

\bigskip

as before but disabled horizontal shift of the charge of \ch{H+}:\par
\ch{@{charge-hshift=0pt} 2 H^+_{(aq)} + CO3^{2-}_{(aq)} -> CO2_{(g)} + H2O_{(l)}}

\end{document}