alignboxesmath-modemathtoolsmhchem
\mbox doesn't work inside align*:
\begin{document}
\usepackage{mhchem}
\begin{align*}
\ce{K_w} &= \ce{[H3O+][OH^-]} \\
-\log \ce{K_w} &= -\log ( \ce{[H3O+][OH^-]} ) \\
-\log \ce{K_w} &= - ( \log \ce{[H3O+]} + \log \ce{[OH^-]} ) & \text{(Using log law for RHS.)}\\
-\log \ce{K_w} &= - \log \ce{[H3O+]} - \log \ce{[OH^-]} \\
-\log \ce{K_w} &= - \log \ce{[H3O+]} + - \log \ce{[OH^-]} \\
\mbox{ \ce{pK_w} &= \ce{pH} + \ce{pOH} }\\
\end{align*}
\end{document}
Edit: mhchem version 3.16 includes the fix shown below. Update will be available in a few days.
(This redefinition requires the current version of mhchem, not the one in texlive 2012)
\documentclass{article}
\usepackage{amsmath}
\usepackage{mhchem}
\makeatletter
\def\mhchem@cee@i#1\\#2#3\mhchem@ENDEND{%
\ifx\mhchem@END#2%
\mhchem@cee@ii#1&\mhchem@END\mhchem@ENDEND%
\else%
\mhchem@cee@ii#1&\mhchem@END\mhchem@ENDEND%
\g@addto@macro\mhchem@ce@out{\\}%
\@ifnextchar[\mhchem@cee@ibrk
\mhchem@cee@i#2#3\mhchem@ENDEND%
\fi%
}
\def\mhchem@cee@ibrk[#1]{%
\g@addto@macro\mhchem@ce@out{[#1]}\mhchem@cee@i}
\makeatother
%with \cee command
\begin{document}
\begin{align}
\cee{
A &-> B \\[10pt] %"[10pt]" is printed, no vertical adjustment
A &-> B
}
\end{align}
%without \cee command
\begin{align}
A &\rightarrow B \\[10pt] %vertical adjustment works
A &\rightarrow B
\end{align}
\end{document}
Don't use align. Use gather instead.
\documentclass[fleqn]{article}
\usepackage{amsmath}
\begin{document}
\begin{gather}
b := (a \oplus s_1) \oplus s_2 \\
e := 0
\end{gather}
\end{document}
Best Answer
Perhaps you are trying to place a box around the last equation. One way to do that is to use
Aboxedfrom themathtoolspackage:While it is true that the descriptions of
mboxtell you that it creates a box just wide enough to hold the text, this is not a box that gets drawn. It is a virtual "box" in the TeX sense that does not get split across lines.