When writing or editing posts, chemistry and math formatting can take up a lot of your time. There is a set of scripts that can help. You can easily add formulas like $\ce{H2O}$ in the text, or in a separated line:
$\ce{2H2(g) + O2(g) -> 2H2O}$
You can also quickly format quantities such as a density of 4.524 g mL-1 or some math:
$$\rho = \pu{4.524 g mL-1}$$
The installation instructions are here: https://stackapps.com/questions/3373/mathjax-buttons
You need to install the script and allow sites to use it (browser dependent). When it is installed, there should be an extra set of commands at the top of the editing box.
There are also keyboard shortcut (Alt + S, M, D, C) instead of pushing buttons. The details are given in the link. To see the userscript in action, watch this two-minute video.