ChemStack is a little HyperCard stack which does quick-and-dirty calculation of formula weight and percentage composition for compounds with 4 or less types of elements in them. Complete documentation is included in the stack.
In order to use ChemStack, you'll need the HyperCard player version 2.2 or better (actually, it might work with 2.1, but I haven't tried). You can get the HyperCard player from any of Apple's anonymous ftp sites; you could try http://mirror.apple.com//mirrors/Apple_SW_Updates/US/Macintosh/Utilities/Hypercard_Player_2.3.5.sea.hqx
ChemStack is freeware; I won't charge anything for it (it doesn't do all that much right now, which is why it's a beta). If you like it, tell me so. My e-mail address is rfredric@midstate.tds.net .
Have fun! And visit my website at http://www.nether.net/~pascual/index.html (if you don't have the index.html, it might not work; heck, it might not work anyway...)