Here's an excellent introduction to solving empirical formula problems with a rhyme. Go to A Simple Rhyme for a Simple Formula for an example and Empirical Formula - Part 2 for sample problems. The University of Wisconsin's Chemical of the Week site has loads of interesting information about lime, chlorine, phosphoric acid, and ammonia. Web Elements on Nitrogen is an excellent source of information on nitrogen. Go to The Nitrogen Family for additional background and data or check out A Car Powered by Liquid Nitrogen.