Align math explanation

Question

Any possibility to align the fractions (units in general) too?

Wobei:
\begin{flalign*}
\noindent \tab \rho _{KOH} & = 2.04 \frac{g}{cm^3} &\\
M_{KOH} & = 56.10 \frac{g}{mol} &\\
c_{KOH} & = 0.05 \frac{mol}{L} &\\
\end{flalign*}

If I add another & they end up completely at the right.

Answer 1

You can use the following:

\documentclass{article}

\usepackage{amsmath}
\usepackage[per-mode = fraction]{siunitx}
\usepackage{chemmacros}

\begin{document}

\begin{alignat*}{2}
\rho_{\ch{KOH}} &=  &  2.04\,&\si{\g\per\cubic\cm} \\
   M_{\ch{KOH}} &={}& 56.10\,&\si{\g\per\mol}      \\
   c_{\ch{KOH}} &=  &  0.05\,&\si{\mol\per\L}
\end{alignat*}

\end{document}

Note: It's a guess that "KOH" is a chemical formula; if not, remove the chemmacros package and write \textup{KOH} instead of \ch{KOH}.

Answer 2

This uses the (in progress) tabstackengine package (introduced at Writing a table with equally spaced columns, based on the widest column, source code at Measuring align). I really need to finish up the documentation to release the code.

\documentclass{article}
\usepackage{tabstackengine}
\stackMath
\setstackgap{S}{1.5ex}
\begin{document}
\noindent Wobei:
\par\medskip\noindent\tabbedShortstack[l]{
\rho _{KOH} &= 2.04 &\frac{\mathrm{g}}{\mathrm{cm}^3} \\
M_{KOH} &= 56.10 &\frac{\mathrm{g}}{\mathrm{mol}} \\
c_{KOH} &= 0.05 &\frac{\mathrm{mol}}{\mathrm{L}} \\
}
\end{document}