Why does everyone expect you to know the PID? Here's the human-friendly way:

pwdx `pgrep ###process_name###`

UPDATED due to David Moles' comment, below.

See other answers for the path to the executable.

In case you need it, this will give you the working directory of the process. You don't need to know the PID:

pwdx `pgrep ###process_name###`