I have officially released the molecular weight/ molar mass calculator that I have been working on for the past few weeks. Type in a chemical formula and out comes the sum of the individual atomic weights of the atoms. You can try it now right here.
It also works for polyatomic ions that require parentheses and hydrates with dots in the formula. It works without any rounding errors (which took a while to put in) and it even displays the number of each atom in the chemical formula.
The basic process the program goes through is this: first, it checks to make sure that the chemical formula does not contain any random characters like !@#$%^&*; next it check to make sure the formula contains only real elements (for example Xi is not an element symbol but it looks like one); then it counts the number of each atom; finally it finds the sum of the atomic weights of the atoms.
If you have a suggestion or a complaint please contact me and I will do what I can.