An incomplete guide to making Wolfram Alpha do what you ask
This is intended to be both a general and specific guide to getting W|A to do what you want, without having to try endless permutations.
The general guides should give hints for most queries, and the domain specific guides should help with getting W|A to realise you're talking about that domain, and how it understands that domain.
(ARTICLE JUST STARTED, PLEASE ADD WHATEVER YOU CAN)
General syntax guide
Wolfram Alpha understands basic Mathematica syntax and function names. See  for details.
Alpha also understands common variations in mathematical syntax. For example, gcd(3 + 4, 21) works just as well as gcd[3 + 4, 21], or even gcd (3 + 4) 21.
Syntax for specific fields
link to a page for each from here
General dumping area for queries that work (eventally!)
(0.209 m3 O2 at 246.8 degrees K and 0.2 atm mass in grams)+ (0.791 m3 N2 at 246.8 degrees K and 0.2 atm mass in grams)