Table aligning to make it more aesthetic!

Here s a small selection of possible layouts:

\documentclass[12pt,a4paper]{report}
\renewcommand{\familydefault}{\rmdefault}
\usepackage[top=1in,bottom=1in,left=1.25in,right=1in]{geometry}
\usepackage[utf8]{inputenc}
\usepackage{booktabs}
\usepackage{siunitx}
\usepackage{multirow}
\usepackage{makecell}
\renewcommand{\theadfont}{\normalsize\bfseries}
\usepackage{calc}
\newlength{\mycolumnwidth}
\setlength{\mycolumnwidth}{\widthof{\textbf{Concentration (mole/L)}}}

\begin{document}
\begin{table}[h]
\sisetup{table-column-width=0.5\mycolumnwidth}
  \centering
  \begin{tabular}{@{}cr@{}c@{}lSS[table-format=2.3]S[table-format=1.3]@{}}
      \toprule
      \multirow{2.5}{*}{\thead{Batch}} &
      \multicolumn{3}{c}{\multirow{2.5}{*}{\thead{Ratio}}} &
      \multicolumn{2}{c}{\thead{Concentration (mole/L)}} \\
      \cmidrule{5-6}
      &&&& {\thead{1}} & {\thead{2}}\\ 
      \midrule
      1 & 10 & : & 1   & 55.55 & 5.555 \\
      2 & 1 & : & 1    & 7.8   & 7.8   \\
      3 & 1 & : & 10   & 0.61  & 6.1   \\
      4 & 1 & : & 1.43 & 1.134 & 0.795 \\ \bottomrule
  \end{tabular}%
\end{table}

\begin{table}[h]
\sisetup{table-column-width=0.5\mycolumnwidth}
  \centering
  \begin{tabular}{@{}cr@{}c@{}lSS[table-format=2.3]S[table-format=1.3]@{}}
      \toprule
      \thead{Batch} &
      \multicolumn{3}{c}{\thead{Ratio}} &
      \multicolumn{2}{c}{\thead{Concentration (mole/L)}} \\
      \cmidrule{5-6}
      &&&& {\thead{1}} & {\thead{2}}\\ 
      \midrule
      1 & 10 & : & 1   & 55.55 & 5.555 \\
      2 & 1 & : & 1    & 7.8   & 7.8   \\
      3 & 1 & : & 10   & 0.61  & 6.1   \\
      4 & 1 & : & 1.43 & 1.134 & 0.795 \\ \bottomrule
  \end{tabular}%
\end{table}

\begin{table}[h]
  \centering
  \begin{tabular}{@{}cr@{}c@{}lSS[table-format=2.3]S[table-format=1.3]@{}}
      \toprule
      \multirow{3.5}{*}{\thead{Batch}} &
      \multicolumn{3}{c}{\multirow{3.5}{*}{\thead{Ratio}}} &
      \multicolumn{2}{c}{\thead{Concentration \\ (mole/L)}} \\
      \cmidrule{5-6}
      &&&& {\thead{1}} & {\thead{2}}\\ 
      \midrule
      1 & 10 & : & 1   & 55.55 & 5.555 \\
      2 & 1 & : & 1    & 7.8   & 7.8   \\
      3 & 1 & : & 10   & 0.61  & 6.1   \\
      4 & 1 & : & 1.43 & 1.134 & 0.795 \\ \bottomrule
  \end{tabular}%
\end{table}

\begin{table}[h]
  \centering
  \begin{tabular}{@{}cr@{}c@{}lSS[table-format=2.3]S[table-format=1.3]@{}}
      \toprule
      \thead{Batch} &
      \multicolumn{3}{c}{\thead{Ratio}} &
      \multicolumn{2}{c}{\thead{Concentration \\ (mole/L)}} \\
      \cmidrule{5-6}
      &&&& {\thead{1}} & {\thead{2}}\\ 
      \midrule
      1 & 10 & : & 1   & 55.55 & 5.555 \\
      2 & 1 & : & 1    & 7.8   & 7.8   \\
      3 & 1 & : & 10   & 0.61  & 6.1   \\
      4 & 1 & : & 1.43 & 1.134 & 0.795 \\ \bottomrule
  \end{tabular}%
\end{table}
\end{document}

I would (a) employ only 4 columns in the tabular environment and (b) load the dcolumn package and create a new column type which aligns its contents on the : (colon) character. I would also question the need to shout at readers by using bold-facing in the header row. A well-designed table gets its message across without having to resort to the visual equivalent of shouting.

\documentclass[12pt,a4paper]{report}
%\renewcommand{\familydefault}{\rmdefault}  %% that's the default
\usepackage[vmargin=1in,hmargin=1in]{geometry}
\usepackage[utf8]{inputenc}
\usepackage{booktabs,siunitx,dcolumn}
\newcolumntype{d}[1]{D{:}{:}{#1}}

\begin{document}
\begin{table}[h]
  \centering
  \begin{tabular}{@{} c d{2.3} S[table-format=2.3] S[table-format=1.3] @{}}
    \toprule
    Batch & \multicolumn{1}{c}{Ratio} & \multicolumn{2}{c@{}}{Concentration} \\
    & & \multicolumn{2}{c@{}}{(mole/L)}\\
    \cmidrule(l){3-4}
    & & {1} & {2} \\ 
    \midrule
      1 & 10:1   & 55.55  & 5.555 \\
      2 & 1:1    &  7.8   & 7.8   \\
      3 & 1:10   &  0.61  & 6.1   \\
      4 & 1:1.43 &  1.134 & 0.795 \\ 
    \bottomrule
  \end{tabular}
\end{table}
\end{document}