Table without thin vertical lines in Tabularx (Table in style of ACS Macro Letter journal)

In this Thread (https://tex.stackexchange.com/a/301036/229635) it was explained how to make the small vertical lines between two cells disappear by using tabularx and columncolor and changing the overhang parameters within a multicolumn.

Now I am looking for a way how to create a Table like shown in the picture attached to this post.

The reason why I am asking for your help is that the code updated on August 7th 2020 by Bernard (see link above) doesn’t create a table fitting to the text width of my document.

In other words: Is there a way to get rid of all the absolute values and the multicolumn used in the m{} parameters so that the table fits still the textwidth no matter how many columns the table has while still implementing the overhang parameter for color fill to avoid the thin white lines between the cells?
I Want to create a Table having no gridlines and a light gray head row without the small vertical lines between color filled cells while using cellcolor.

Thanks a lot in advance,
Benson_G

Picture Reference:
Worm, M.; Leibig, D.; Dingels, C.; Frey, H. Cleavable Polyethylene Glycol: 3,4-Epoxy-1-butene as a Comonomer to Establish Degradability at Physiologically Relevant pH.ACS Macro Letters2016,5(12), 1357–1363

documentclass[a4paper,10pt, enabledeprecatedfontcommands]{scrartcl} %Art des Dokuments, Schriftgröße etc.
usepackage[utf8]{inputenc}%Direkte Angabe von Umlauten möglich 
usepackage[left=3cm,right=2cm,top=2cm,bottom=2cm,includehead]{geometry}%Maße für die wissenschaftliche Arbeit
usepackage[parfill]{parskip} %Bessere Seitenumbrüche
usepackage[onehalfspacing]{setspace} %Zeilenabstand
usepackage[T1]{fontenc} %Silbentrennung
usepackage[english]{babel} %Sprachanpassungen
usepackage{graphicx} %Einfügen von Bildern
usepackage{tabularx} %Tabellen
usepackage{booktabs} %Weitere Möglichkeiten für Tabellen
usepackage{here} %Lage von Tabellen und Bildern 
usepackage{units}
usepackage{physics}
usepackage{amsmath}
usepackage{textgreek}
usepackage{textcomp}
usepackage{amssymb}
usepackage[labelformat=simple]{subcaption}
usepackage{float}
usepackage{pdfpages}
usepackage{lmodern} 
usepackage{enumitem}
usepackage[hyphens]{url}  %% be sure to specify the option 'hyphens'
% usepackage[pdftex,bookmarksnumbered,hidelinks,breaklinks]{hyperref}
usepackage[superscript]{cite}
usepackage[scaled]{helvet}
renewcommandfamilydefault{sfdefault} 
usepackage[version=3]{mhchem} %Summenformeln angeben
usepackage[labelsep=period]{caption}
usepackage[per-mode = reciprocal, output-decimal-marker={,}, exponent-product = {cdot}]{siunitx} %Si-Einheiten
% usepackage[colorlinks,
% pdfpagelabels,
% pdfstartview = FitH,
% bookmarksopen = true,
% bookmarksnumbered = true,
% linkcolor = black,
% plainpages = false,
% hypertexnames = false,
% citecolor = black]{hyperref}
usepackage{color}
usepackage{colortbl}

definecolor{hellgrau}{rgb}{0.90,0.90,0.90}

newcommand{RM}[1]{MakeUppercase{romannumeral #1{.}}}

renewcommandthesubfigure{thefigure.arabic{subfigure}}
addtokomafont{captionlabel}{bfseries}

DeclareNewTOC[
  type=scheme,                         % Name der Umgebung
  types=schemes,                       % Erweiterung (listofschemes)
  float,                               % soll gleiten
  floatpos=tbp,                        % voreingestellte Gleitparameter
  name=Scheme,                         % Name in Überschriften
  listname={Verzeichnis der Schemata}, % Listenname
                                       % counterwithin=chapter
]{los}

begin{document}

begin{table}[H]
vspace{-1.8 baselineskip}
  centering
  setlengthbelowcaptionskip{5 pt}
  caption{textbf{Reaction mixture used for the synthesis of cyclohexanone 4-nitrophenylhydrazone.}}
    begin{tabular}{cccccc}  
     cellcolor{hellgrau} chemicals   &cellcolor{hellgrau} textit{M} $^a$ (g mol$^{text{-1}}$)  &cellcolor{hellgrau} textit{n} $^a$(mol 10$^{text{-4}}$)  &cellcolor{hellgrau} textit{V} $^a$ (mL) &cellcolor{hellgrau}  textit{m} $^a$ (g) &cellcolor{hellgrau} $varrho$ $^a$ (g mL$^{text{-1}}$)
    Cyclohexanone               & 98.14   &  multicolumn{4}{c}{0.5 mL of the destillate}    
    4-Nitrophenyl hydrazine     & 153.14  &  26    &   -  & 0.4   & -     
    Sulfuric acid               & 98.08   &  372   &   2  & 3.68  & 1.84 
    Water                       & 18.02   & 1664   &   3  & 3     & 1.00 
    Ethanol                     & 47.07   & 1678   &   10 & 7.9   & 0.79    
    end{tabular} 
    vspace{0.6 baselineskip} 
    raggedright
    $^a$Molecular mass textit{M}, amount of substance textit{n}, volume textit{V}, mass textit{m},  density $varrho$.
  label{tab:ReactionMixtureCyclohexanone4-nitrophenylhydrazone}%
  vspace{-1.8 baselineskip}
end{table}

end{document}