What is the most powerful acid?
Fluoroantimonic acid
Fluoroantimonic acid is an inorganic compound with the chemical formula H 2FSbF 6 (also written H 2F[SbF 6], 2HF·SbF5, or simply HF-SbF5). It is an extremely strong acid, easily qualifying as a superacid. The Hammett acidity function, H0, has been measured for different ratios of HF:SbF5. While the H0 of pure HF is −15, addition of just 1mol % of SbF5 lowers it to around −20. However, further addition of SbF5 results in rapidly diminishing returns, with the H0 reaching −21 at 10mol %. The use of an extremely weak base as indicator shows that the lowest attainable H0, even with > 50mol % SbF5, is somewhere between −21 and −23.[1][2][3]
Leveling effect
The same argument applies to bases. In water, OH is the strongest base. Thus, even though sodium amide (NaNH) is an exceptional base (pK of NH ~ 33), in water it is only as good as sodium hydroxide. On the other hand, NaNH is a far more basic reagent in ammonia than is NaOH.