center text in part of tabularx table

Question

I wanted to ask how I center the text in the description row (not in the rest of the table that will follow. This to reduce the space between the text and the \cmidrule

\documentclass[a4paper, 11pt]{article}
\usepackage[left=2cm,top=2cm,right=2cm, bottom=2cm]{geometry}
\usepackage[utf8]{inputenc} 
\usepackage{caption}
\usepackage{siunitx}
\usepackage{ragged2e}
\usepackage{booktabs, tabularx} 
\newcolumntype{C}{>{\RaggedRight\hspace{0pt}\arraybackslash}X}
\newcolumntype{P}[1]{>{\RaggedRight\hspace{0pt}}p{#1}}
\newlength\colwidth
\usepackage{float}  
\usepackage[version=4]{mhchem}


\begin{document}


 \begin{table}[H]
\settowidth\colwidth{Solutionn} 
\begin{tabularx}{\textwidth}{@{}P{\colwidth}*  {7}{C} @{}}
\toprule 
\textbf{Solution}     &      \multicolumn{3}{c}{\textbf{amount of substance [$\si{\mol}$]}  }     &       \multicolumn{3}{c}{\textbf{mass of substance [$\si{\gram}$]}  }     &\textbf{Volume of Stock [$\si{\milli\liter}$]}      \\
\cmidrule(lr){2-4} \cmidrule(lr){5-7}
 & $\ce{KH2PO4}$ & $\ce{Na2HPO4}$ & $\ce{NaCl}$ & $\ce{KH2PO4}$ & $\ce{Na2HPO4}$ & $\ce{NaCl}$\\
\midrule
A  \\
\bottomrule
\end{tabularx}
\endtabularx
\end{table}

\end{document}

Answer 1

I think that you looking for the following:

By use of \multirow package you can define cell which span two (or more) text lines in the adjacent cells in spanned rows. For horisontal centering of text in some cells, you can use \multicolumn{1}{c}{...} command.

In MWE below are also considered @Mico comments below your question:

\documentclass[a4paper, 11pt]{article}
\usepackage[margin=2cm]{geometry}
\usepackage{caption}
\usepackage{siunitx}
\usepackage{ragged2e}
\usepackage{booktabs, multirow, tabularx}
\newcolumntype{L}[1]{>{\RaggedRight\hspace{0pt}\arraybackslash%
                     \hsize=#1\hsize}X}
\newcolumntype{P}[1]{>{\RaggedRight\hspace{0pt}}p{#1}}
\NewExpandableDocumentCommand\mcc{O{c}m}{\multicolumn{1}{#1}{#2}}
\newlength\colwidth
\usepackage{float}
\usepackage[version=4]{mhchem}

\begin{document}
    \begin{table}[ht]   % <---
    \settowidth\colwidth{Solutionn}
    \setlength\tabcolsep{3pt}
\begin{tabularx}{\textwidth}{@{} P{\colwidth} *{6}{L{0.98}} L{1.12} @{}}
    \toprule
\multirow{2.4}{=}{\textbf{Solution}}
    & \multicolumn{3}{c}{\textbf{amount of substance [\si{\mol}]} }     
        & \multicolumn{3}{c}{\textbf{mass of substance [\si{\gram}]} }     
            & \multirow{2.4}{=}{\textbf{Volume of Stock [\si{\milli\liter}]}}\\
    \cmidrule(lr){2-4} \cmidrule(lr){5-7}
    & \mcc{\ce{KH2PO4}} & \mcc{\ce{Na2HPO4}} & \mcc{\ce{NaCl}} 
    & \mcc{\ce{KH2PO4}} & \mcc{\ce{Na2HPO4}} & \mcc{\ce{NaCl}}    &    \\
    \midrule
A  &    &   &   &   &   &   &                       \\
    \bottomrule
\end{tabularx}
%\endtabularx % superfluous 
    \end{table}
\end{document}

Answer 2

It's handy to create a shortcut macro, say,

\newcolumntype{C}{>{\Centering\hspace{0pt}}X}

and write \mC{\ce{KH2PO4}} to override the default formatting for that cell only.

\documentclass[a4paper, 11pt]{article}
\usepackage[margin=2cm]{geometry}
\usepackage[utf8]{inputenc} 
\usepackage[T1]{fontenc}
\usepackage{caption,siunitx,ragged2e,booktabs,tabularx} 
\newcolumntype{L}{>{\RaggedRight\hspace{0pt}}X}
\newcolumntype{C}{>{\Centering\hspace{0pt}}X}
\newcolumntype{P}[1]{>{\RaggedRight\hspace{0pt}}p{#1}}
\usepackage[version=4]{mhchem}

\newlength\colwidth
\newcommand\mC[1]{\multicolumn{1}{C}{#1}} % handy shortcut macro

\begin{document}

\begin{table}[ht!]
\settowidth\colwidth{\textbf{Solution}} % <-- need to use \textbf to get correct measurement
\begin{tabularx}{\textwidth}{@{} P{\colwidth} *{7}{L} @{}}
\toprule 
\textbf{Solution} &  
\multicolumn{3}{c}{\textbf{Amount of substance} [\si{\mol}]}   &       
\multicolumn{3}{c}{\textbf{Mass of substance} [\si{\gram}]}   &
\multicolumn{1}{C@{}}{\textbf{Volume of stock} [\si{\milli\liter}]}      \\
\cmidrule(lr){2-4} \cmidrule(lr){5-7}
& \mC{\ce{KH2PO4}} & \mC{\ce{Na2HPO4}} & \mC{\ce{NaCl}}
& \mC{\ce{KH2PO4}} & \mC{\ce{Na2HPO4}} & \mC{\ce{NaCl}} \\
\midrule
A \\
B \\
\bottomrule
\end{tabularx}
\end{table}

\end{document}