Error with \usepackage{mhchem}. It was working and now it cannot compile

Question

The whole report cannot even compile now! And the pdf cannot open any more! I narrowed down the problem to \usepackage{mhchem}. I was using version 3 (if it matters) but i also tried writing version=4. I read here that there is an issue when updating latex. So i tried uninstalling and installing the previous version that was working and still nothing. An example:

\documentclass{book}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage[version=4]{mhchem}

\begin{document}
    \begin{equation}
        \label{eq:HDO}
        \ce{C$H_{1.4}O_{0.4}$ -> 1''C$H_{2}$ '' + 0.4H2O}
    \end{equation}
\end{document}

It does nothing. trying to finish compile or just say:

Undefined control sequence. \begin{document}.

Answer 1

You get an error Unexpected input character for the '. They are not supported (yet). But in your example, I don't even know what they should represent. What is 1''?

By the way, this input definitely did never work with any version of mhchem.

You don't need the $, just remove them.

\ce{CH_{1.4}O_{0.4} -> 1CH2 + 0.4H2O} works just fine. (Although 2+0.4*2=2.8, not 1.4.)

Unfortunately, up to v4.04 (which seems to be included in TeX Live 2016, from reports I get) there was a bug, causing an infinite loop of Unexpected input character when running in nonstop mode, as is the case when compiling within TeX editors.