Question

How do I determine the pKa of a molecule by looking at it?

Answer 1

I'm not convinced this can be done. Here is a page on the pKa calculation:
http://www.webassign.net/marvin/help/calculations/pKa.html

Please correct me if I'm wrong, but I'm pretty sure that ion activity needs to be determined experimentally.

For what it is worth, the following relation may be useful:\[14.00=pK_a+pK_b\]

Answer 2

you have to compare it to a known pka and then use some acidity rules to see if its more acidic or less acidic than the others if you need help message me on facebook.