If you insist using vertical and horizontal rules, vertically and horizontal spanned cells and similar, you should take a look at the package cals. Very easy to set up.
In addition, I have used a KOMA-script class, because it has sensible margins and internal commands to change captions.
The way I have set up the calstable, it may span multiple pages. It is not floating, and I use
\captionaboveof{table}{My wide table\label{tab:wide}}
to have a correctly formatted caption. The \label-command goes inside the caption command, so it is not lost.
The best is of cause to get rid of most of the rules, an using booktabs and additional spacing to group rows and columns to make the tabular more readable.
\documentclass[british, DIV=12, captions=tableheading]{scrartcl}
\usepackage{cals, url}
\usepackage{babel}
\usepackage{siunitx}
\usepackage[version=4]{mhchem}
\usepackage[T1]{fontenc}
\let\nc=\nullcell % Shortcuts
\let\sc=\spancontent
\addtokomafont{caption}{\sffamily\bfseries\small}
\setkomafont{captionlabel}{\normalfont}
\begin{document}
\bigskip
\begin{calstable}[c]
\captionaboveof{table}{My wide table\label{tab:wide}}
% Defining columns relative to each other and relative to the margins
\colwidths{{\dimexpr(\columnwidth)/10\relax}
{\dimexpr(\columnwidth)/10\relax}
{\dimexpr(\columnwidth)/10\relax}
{\dimexpr(\columnwidth)/10\relax}
{\dimexpr(\columnwidth)/10\relax}
{\dimexpr(\columnwidth)/10\relax}
{\dimexpr(\columnwidth)/10\relax}
{\dimexpr(\columnwidth)/10\relax}
{\dimexpr(\columnwidth)/10\relax}
{\dimexpr(\columnwidth)/10\relax}
}
% The tabular fills the text area if sum of all columns is 10
% Set up the tabular
\makeatletter
\def\cals@framers@width{0.8pt} % Outside frame rules, reduce if the rule is too heavy
\def\cals@framecs@width{0.4pt}
\def\cals@bodyrs@width{0.6pt}
\def\cals@cs@width{0.4pt} % Inside rules, reduce if the rule is too heavy
\def\cals@rs@width{0.6pt}
\def\cals@bgcolor{}
\setlength{\cals@paddingL}{3.2pt}
\setlength{\cals@paddingR}{3.2pt}
% R1H1
\thead{\sffamily\small% The first 3 rows are heading and vill be repeated on every page
\brow
\nc{lrt}
\nc{ltb}
\nc{tb}
\nc{tb}
\nc{tb}
\nc{tb}
\nc{tb}
\nc{tb}
\nc{tb}
\nc{rtb}\alignC\sc{Volume of gas produced (\si{cm^3 \pm 1cm^3})}
\erow
%R2H2
\brow
\nc{lr}
\nc{ltb}
\nc{tb}
\nc{rtb}\sc{\SI{0.42}{mol/dm^3} \ce{C2H4O2} and \SI{0.4}{mol/dm^3} of \ce{NaHCO3}}
\nc{ltb}
\nc{tb}
\nc{rtb}\sc{\SI{0.42}{mol/dm^3} \ce{C2H4O2} and\\ \SI{0.4}{mol/dm^3} of \ce{NaHCO3}}
\nc{ltb}
\nc{tb}
\nc{rtb}\sc{\SI{0.84}{mol/dm^3} \ce{C2H4O2} and\\ \SI{0.8}{mol/dm^3} of \ce{NaHCO3}}
\erow
%R3H3
\brow
\nc{lrb}\alignC\sc{\vfil Time elapsed \si{s \pm 1 s}}
\bfseries\cell{Trial 1}
\cell{Trial 2}
\cell{Trial 3}
\cell{Trial 1}
\cell{Trial 2}
\cell{Trial 3}
\cell{Trial 1}
\cell{Trial 2}
\cell{Trial 3}\normalfont
\erow
}
\tfoot{\lastrule\strut}
%R4B1
\brow
\cell{15}
\cell{20}
\cell{22}
\cell{18}
\cell{25}
\cell{15}
\cell{17}
\cell{5}
\cell{5}
\cell{6}
\erow
%R5B2
\brow
\cell{30}
\cell{27}
\cell{29}
\cell{25}
\cell{31}
\cell{25}
\cell{22}
\cell{8}
\cell{8}
\cell{8}
\erow
%R6B3
\brow
\cell{45}
\cell{32}
\cell{33}
\cell{29}
\cell{35}
\cell{32}
\cell{28}
\cell{10}
\cell{11}
\cell{11}
\erow
%R7B4
\brow
\cell{60}
\cell{35}
\cell{36}
\cell{33}
\cell{38}
\cell{38}
\cell{34}
\cell{12}
\cell{13}
\cell{12}
\erow
%R8B5
\brow
\cell{75}
\cell{38}
\cell{38}
\cell{36}
\cell{41}
\cell{42}
\cell{38}
\cell{14}
\cell{15}
\cell{14}
\erow
%R9B6
\brow
\cell{90}
\cell{41}
\cell{42}
\cell{38}
\cell{43}
\cell{46}
\cell{42}
\cell{15}
\cell{17}
\cell{16}
\erow
%R10B7
\brow
\cell{105}
\cell{42}
\cell{44}
\cell{41}
\cell{44}
\cell{50}
\cell{46}
\cell{16}
\cell{19}
\cell{17}
\erow
%R11B8
\brow
\cell{120}
\cell{44}
\cell{45}
\cell{43}
\cell{46}
\cell{53}
\cell{48}
\cell{18}
\cell{21}
\cell{18}
\erow
%R12B9
\brow
\cell{135}
\cell{46}
\cell{47}
\cell{44}
\cell{47}
\cell{55}
\cell{51}
\cell{18}
\cell{22}
\cell{20}
\erow
%R13B10
\brow
\cell{150}
\cell{48}
\cell{49}
\cell{46}
\cell{49}
\cell{58}
\cell{54}
\cell{19}
\cell{24}
\cell{22}
\erow
%R14B11
\brow
\cell{165}
\cell{50}
\cell{51}
\cell{47}
\cell{50}
\cell{60}
\cell{56}
\cell{20}
\cell{25}
\cell{23}
\erow
%R15B12
\brow
\cell{180}
\cell{51}
\cell{52}
\cell{49}
\cell{51}
\cell{61}
\cell{58}
\cell{21}
\cell{26}
\cell{25}
\erow
\makeatletter
\end{calstable}
\end{document}
chemformulaormhchemor oder to typeset the chemical formulae instead of using math mode for this task.siunitxfor the units might also come in handy.