chem.py is a set of tools that help make high school chemistry calculations less tedious.
Go to the green Clone or download button, click it, then click Download ZIP. Then, unpack the .zip, including all files, to some folder.
As always, there's the Cool Kid option of cloning this git repo. That is preferable, since you can update simply by pulling this repo.
python chem.py ...
chem.py depends on Python 3.6 and above.
Note that the command to invoke Python may be different (e.g. python3), so adjust accordingly.