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

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


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

Undefined control sequence. \begin{document}.

• (1) welcome, (2) why are you using those $in math mode? And what does _{1.4} of hydrogen actually mean. – daleif Nov 28 '16 at 16:03 • On a recent TeX installation, I don't get the error "undefined control sequence" but complaints from the mhchem package about characters that cannot be interpreted. The complaints go away when I remove the $ signs (math mode is not required for subscripts to work) as well as the quotes ''. – gernot Nov 28 '16 at 16:06

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''?
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.