For some strange reason, the right column is left aligned in contrast with the other columns. I'm not seeing what I did wrong. This is due in 3 hours, but I haven't noticed it until now. Please help if you can. Very much appreciated!
\documentclass[letterpaper]{article}
\special{papersize=8.5in,11in}
\usepackage{mathtools}
\usepackage[version=4]{mhchem}
\usepackage{siunitx}
\usepackage{booktabs}
\usepackage{array}
\usepackage[margin=1in]{geometry}
\setlength{\parskip}{1em}
\begin{document}
\begin{table}[h!]
\renewcommand\arraystretch{2}
\begin{center}
\caption{Processed data (rate of reaction)}
\vspace{.5 em}
\begin{tabular}{@{}lccl@{}}
\toprule
& \renewcommand\arraystretch{1.5} \begin{tabular}[c]{@{}c@{}} moles of $\ce{Mg_{(s)}} \hspace{1mm} \left( \si{mol} \right)$ \\ $\left( \pm \hspace{1mm} 20 \% \right)$ \end{tabular} & \renewcommand\arraystretch{1.5} \begin{tabular}[c]{@{}c@{}}time $\left( \si{s} \right)$ \\ $\left( \pm 0.01 \hspace{1mm} \si{s} \right)$ \end{tabular} & \renewcommand\arraystretch{1.5} \begin{tabular}[c]{@{}c@{}} reaction rate $ \left( \si{mol.s^{-1}} \right)$ \\ $\left( \pm \hspace{1mm} 20 \% \right)$ \end{tabular} \\
\midrule \renewcommand\arraystretch{2}
one piece of $\ce{Mg}$ ribbon & $2 \times 10^{-3}$ & 17.04 & $1 \times 10^{-4}$ \\
small cut up pieces of $\ce{Mg}$ & $2 \times 10^{-3}$ & 16.03 & $1 \times 10^{-4}$ \\
\bottomrule
\end{tabular}
\end{center}
\end{table}
\end{document}
I've put my preamble and the entire table.
Edit: code
Here's a picture.
lccl
?lccc
if you want it to be centred.center
environment, but simply\centering
to prevent the additional vertical spacing.