system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5 fluoroantimonic acid ph

What is the Ph of this super acid? And what makes this acid so much stronger than any other acid? How much stronger is it than stomach acid?
gpt
pH is kind of a useless way to talk about acidity because it depends on the concentration of the acid in its solution. A better metric is called pKa. Without getting into detail, the more negative the pKa is, the stronger an acid is. And it's a logarithmic scale (meaning that if something has a pKa that's higher by 1, then it's actually 10 times more acidic. Higher by 2, then 100x more acidic etc.). The pKa of stomach acid (HCl) is -6.3. The pKa of fluoroantimonic acid is -25. So it's about 19 pKa points more acidic, or 10 000 000 000 000 000 000 (ten quintillion) times more acidic than stomach acid.

The reason is because you have a positively charged fluorine. Now, fluorine really  *really*  likes electrons. In fact, more so than any other atom. Electrons are negatively charged, so if we've got a positively charged fluorine, that means we robbed it of electrons. And it really,  *really*  **really**  wants them back. An acid is really defined as anything that wants to take electrons, and the strength of an acid is ultimately defined as how badly it wants those electrons. Since positively charged fluorine really  *really*  **really**  wants those electrons, it is an incredibly strong acid.

EDIT: Since there seems to be some questions regarding why we're even talking about positively charged fluorines, I'll explain that here. The antimony part of fluoroantimonic acid comes from the addition of SbF5 (antimony pentafluoride) to twice as much HF. SbF5 is strong enough to rip the fluoride from HF, leaving H+. But remember, we've got only half as much SbF5 as HF, so the extra HF is left with the job of taking on the orphaned H+ ions. This leaves you with SbF6- and H2F+, which is fluoroantimonic acid.