Re: Just how strong is fluoroantimonic acid?

The Guinness Book of Records that you were looking at may be a bit out of 
date? The information that I have is that the strongest known acid is a 
mixture of antimony pentafluoride, fluosulfonic acid, and sulfur trioxide. 
Even my information is not the latest.

The pKa scale that we usually use for measuring acidity becomes quite 
meaningless for super-acids -- they are all just as strong as one another 
in aqueous solution, being completely dissociated. It is only when we go to 
other solvents that we can tell the differences between weaker strong acids 
like HCl and stronger strong acids like H2SO4 and HClO4

There is a number that can be associated with the strength of a super-acid, 
known as the Hammett acidity function. Its value is

11.0 for sulfuric acid (H2SO4)
12.6 for fluosulfonic acid (HSO3F)
15.2 for fluoantimonic acid (2 HF + SbF5 -- probably H2F(+)SbF6(-) )
more than 16 for fluosulfonic acid plus sulfur trioxide plus antimony 
pentafluoride.

The reference is Greenwood & Earnshaw, Chemistry of the Elements, pages 57 
and 664. This is a 1989 edition, so it is probably not the last word.