Use \left( and \right) in mhchem

I am still not quite sure in which context you want to display the concentration of Pb2+ ions (inside of the text or of a mathematical equation or...). Nevertheless, here are some options :

\documentclass{article}
\usepackage[version=4]{mhchem}

\usepackage{siunitx} % provides the SI command to typeset number and their units
\sisetup{per-mode=fraction}

\usepackage{chemmacros}
\usechemmodule{units} % Provides the \Molar unit
\begin{document}

\ce{Pb^{2+}} \(\left(c = \SI{1}{\mol\per\liter}\right)\)
  
\[ \ce{Pb^{2+}} \left(c = \SI{1}{\mol\per\liter}\right) \]


\sisetup{per-mode=symbol}
\ce{Pb^{2+}} \((c = \SI{1}{\mol\per\liter})\)

\sisetup{per-mode=reciprocal}
\ce{Pb^{2+}} \((c = \SI{1}{\mol\per\liter})\)

\ce{Pb^{2+}} \((c = \SI{1}{\Molar})\)

\end{document}

The argument of \ce is processed in a quite complicated fashion and math mode is not in effect. This works, see page 9 of the manual.

\documentclass{article}
\usepackage[version=4]{mhchem}

\begin{document}

\[
\ce{
  Zn/Zn^{2+} $\Big($ c = 1\frac{mol}{l} $\Big)$
  // Pb^{2+} $\Big($ c = 1\frac{mol}{l} $\Big)$/Pb
}
\]

\end{document}