How do you Type Letters into Text Boxes on Visual Basic 6.0

Hello,

I am putting together a program, and I was wondering if it was possible to put letters into text boxes, for them to eventually come out as a number after calculation purposes.

for example, I want to find the formula weight of pure Oxygen (O2) Is there a way that I can make the program so that all the user has to do is type O into the text box and that the program will recognize that O stands for 16??

If someone could help itd be greatly appreciated!

Thanks,
Fred