\frac not working

Question

I'm new here so bear with me if I don't catch on to something immediately.

I'm trying to write some script for some copyediting I'm doing and it automatically converts the code into the format once you save the edit (LaTeX and MathJax). However, I'm using a seperate cogeditor to prepare the code and it's fighting me on \frac. It's confusing me, because it literally runs fine later in the code, but the first time it's used it fights me. What's going on?

C_{3} H_{8(g)} + 5O_{2(g)}  \longrightarrow  3CO{2(g)} + 4H_{2}O_{(l)}

\vspace{10pt}

 \Delta H_{rxn} =  \Sigma n \Delta H_{f \hspace{5pt} products} -  \Sigma  m \Delta H_{f\hspace{5pt} reactants} 

\vspace{10pt}

test line

 \frac{kJ}{mol} 

test line


\vspace{10pt}

(-2220.1\frac{kJ}{mol}) \hspace{3pt}= \hspace{3pt}[(3 mol \hspace{3pt} \times\hspace{3pt} -393.5\frac{kJ}{mol}) + (4 mol \hspace{3pt} \times \hspace{3pt} -285.8\frac{kJ}{mol})] - [(1 mol  \hspace{3pt}\times \hspace{3pt} ∆Hf C3H8) + (5 mol\hspace{3pt}  \times \hspace{3pt} 0\frac{kJ}{mol})]

\vspace{10pt}

-2220.1\frac{kJ}{mol} \hspace{3pt}= \hspace{3pt} (-2321.7\frac{kJ}{mol})\hspace{3pt} -\hspace{3pt} ( \Delta H_{f\hspace{3pt} C_{3}H_{8}})

\vspace{10pt}

 \Delta H_{f\hspace{3pt} C_{3}H_{8}} \hspace{3pt}= \hspace{3pt} -103.6\frac{kJ}{mol}

Answer 1

The following example uses specialized packages for the different tasks:

• mhchem for easier input and typesetting of chemical formulas.
• siunitx for setting units, numbers and numbers with units. There are many configuration options.
• amsmath for more advanced features in the area of mathematical typesetting.

Example:

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

\sisetup{per-mode=fraction}

\DeclareSIUnit{\kJm}{\kilo\joule\per\mol}

\begin{document}
\ce{C3H8_{(g)} + 5O2_{(g)} -> 3CO2_{(g)} + 4H_2O_{(l)}}

\vspace{10pt}

\[
  \Delta H_{\text{rxn}} =  \sum_n \Delta H_{\!f\,\text{products}}
  - \sum_m \Delta H_{\!f\,\text{reactants}}
\]

\vspace{10pt}

test line

per-mode=fraction: \si{\kilo\joule\per\mol}

per-mode=symbol: \si[per-mode=symbol]{\kilo\joule\per\mol}

per-mode=reciprocal: \si[per-mode=reciprocal]{\kilo\joule\per\mol}

\vspace{10pt}

\begin{align*}
  \SI{-2220.1}{\kJm}
  ={}& \left[ \SI{3}{\mol} \times \Bigl(\SI{-393.5}{\kJm}\Bigr)
         + \SI{4}{\mol} \times \Bigl(\SI{-285.8}{\kJm}\Bigr)
       \right]
  \\
  &  - \left[ \SI{1}{\mol} \times \Delta H_{\!f\,\ce{C3H8}}
        + \SI{5}{\mol} \times \SI{0}{\kJm}
       \right]
\end{align*}
\end{document}