Math & Science ⇒ Multiple Alignment Points in a System of Equations

[svend_tveskaeg]

Hi all.

Consider the following example:

Code: Select all

\documentclass{article}

\usepackage[danish]{babel}
\usepackage{mathtools}
\usepackage[version=3]{mhchem}

\newcommand{\imp}{\qquad \Longrightarrow \qquad}

\begin{document}

\begin{alignat}{2}
  \left.
  \begin{alignedat}{3}
    K_{\textup{v}} &= [\ce{H+}] \cdot [\ce{OH-}] &&\Rightarrow& [\ce{OH-}] &= \frac{K_{\textup{v}}}{[\ce{H+}]}\\
    [\ce{H+}]      &= [\ce{OH-}] + [\ce{$S$-}]   &&\Rightarrow& [\ce{OH-}] &= [\ce{H+}] - [\ce{$S$-}]
  \end{alignedat}
  \right\}
  &\imp&
  a &= b\\
  &\imp&
  c &= d.
\end{alignat}

\end{document}

I would like to align at both equal signs and at \Rightarrow. In the example above, the spacing between \Rightarrow and [\ce{OH-}] is wrong.

How do I fix this?

Thank you in advance!

[cgnieder]

You have to keep in mind what a & means in what place:

Code: Select all

\begin{alignedat}{6}
 &(middle of first) &(end of first) &(middle of second) &(end of second) &(middle of third)
\end{alignedat}

So this should do the trick:

Code: Select all

\documentclass{article}

\usepackage[danish]{babel}
\usepackage{mathtools}
\usepackage[version=3]{mhchem}

\newcommand{\imp}{\qquad \Longrightarrow \qquad}

\begin{document}

\begin{alignat}{2}
  \left.
  \begin{alignedat}{3}
    K_{\textup{v}} &= [\ce{H+}] \cdot [\ce{OH-}] &&\Rightarrow [\ce{OH-}] &&= \frac{K_{\textup{v}}}{[\ce{H+}]}\\
    [\ce{H+}]  &= [\ce{OH-}] + [\ce{$S$-}]   &&\Rightarrow [\ce{OH-}] &&= [\ce{H+}] - [\ce{$S$-}]
  \end{alignedat}
  \right\}
  &\imp&
  a &= b\\
  &\imp&
  c &= d.
\end{alignat}

Regards

[svend_tveskaeg]

Thank you very much.