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.