Aligning a row of explanations for equation

Question

I was advised to repost the question. I try and give as much input as possible.

Currently I use this code

\begin{flalign*}
\noindent \tab \rho \textsubscript{KOH} & = \num{2.04}\,\si{\g\per\cubic\cm} &\\[-1pt]
\text{M}\textsubscript{KOH} & =  \num{56.10}\,\si{\g\per\mol} &\\[-1pt]
\text{c}\textsubscript{KOH} & =  \num{0.05}\,\si{\mol\per\L}\\
\end{flalign*}

This results in this

I want the = and fractions to be aligned. Svend gave me in my older question the following code

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

Works like a charme for the units and alignment. However the explanations is centered then. Fleqn will left align all my equations, right? Equations should stay centered. I just would like to add the explanations with a little indent and left aligned.

Also is it possible to make the fractions smaller to avoid those big spaces? If anyone knows what I mean by adding explanations and has a even cooler theme for it I'm open to it. I just want it to look neat in my papers/lab reports.

PS: I am aware that the equation is wrong.

Answer 1

I'd define a specific environment:

\documentclass{article}
\usepackage{amsmath,siunitx,chemmacros}
\sisetup{per-mode=fraction}

\newenvironment{wobei}[1][Wobei]
 {#1:\\*[\medskipamount]% <--- Adjust at will
  \hspace*{2em}%             <--- Adjust at will
  $\!\begin{alignedat}{2}}
 {\end{alignedat}$\par\addvspace{\belowdisplayskip}}

\begin{document}
\[
A+B=C
\]
\begin{wobei}
\rho_{\ch{KOH}} &= \num{2.04}  &&\,\si{\g\per\cubic\cm} \\
M_{\ch{KOH}}    &= \num{56.10} &&\,\si{\g\per\mol}      \\
c_{\ch{KOH}}    &= \num{0.05}  &&\,\si{\mol\per\L}
\end{wobei}
\end{document}

My equation is not as wrong as yours. ;-)

You can also call the environment as

\begin{wobei}[Whatever]

and “Whatever:” will be printed instead of “Wobei:”

---

Following Barbara Beeton's idea in comments, here's a different implementation that allows you to align numbers to their decimal point.

\documentclass{article}
\usepackage{amsmath,siunitx,chemmacros,array}
\sisetup{per-mode=fraction}

\newenvironment{wobei}[2][Wobei]
 {\renewcommand{\arraystretch}{2}%
  #1:\\*[\medskipamount]
  \hspace*{2em}\begin{tabular}{
    @{}
    >{$}r<{$}
    @{${}={}$}
    S[table-format=#2]
    @{\thinspace}
    >{\everymath{\displaystyle}}l
    @{}
  }}
 {\end{tabular}\par}

\begin{document}
\[
A+B=C
\]
\begin{wobei}{2.2}
\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{wobei}
\end{document}

Here we have to add a mandatory argument, that is, the number of digits before and after the decimal point in the values. Notice that the equal sign is part of the environment so it shouldn't be typed.

Answer 2

Here is a solution using flalign*; the idea is to use 3 alignments, the first being empty, and the last one for the explanations – if I understood what you require.

\documentclass[ a4paper]{article}
\usepackage[utf8]{inputenc} 
\usepackage[T1]{fontenc} 
\usepackage{lmodern} 
\usepackage[showframe, textwidth = 15cm, nomarginpar, noheadfoot]{geometry} 
\usepackage{mathtools}
\usepackage[per-mode = fraction]{siunitx}
\newcommand\koh{\textsc{\scriptsize koh}}

\begin{document}

This results in this : 
   \begin{flalign*}
      & & c_{\scriptscriptstyle FS} & = \frac{V_{\scriptscriptstyle KOH} \cdot ρ_  {\scriptscriptstyle KOH} }{M_\mathrm{\scriptscriptstyle KOH}} \cdot \frac{100}{V_\mathrm  {{\scriptscriptstyle L}sg}} \cdot c_\mathrm{\scriptscriptstyle KOH}\\
   \shortintertext{Wobei: }
      \mathrlap{\qquad\begin{aligned}
      \rho_\koh & =\hphantom{0}\num{2 .04}\,\si{\g\per\cubic\cm}  \\%
      \mathrm{M}_\koh & =  \num{56.10}\,  \si{\g\per\mol} \\[1pt]
      \mathrm{c}_\koh & = \hphantom{0} \num{0.05}\,  \si{\mol\per\L}
      \end{aligned}}
   \end{flalign*}

\end{document}​