\documentclass[12pt]{article}
\thispagestyle{empty}
\usepackage{hyperref}
\usepackage{amsmath}
\begin{document}
\huge{
$[S_0] = \left(\frac{1}{K_{\rm equilibrium}} \frac{1}{p_A} + 1\right)[A_{\rm adsorption}]$
}
\end{document}