How are the formulas for acids determined?
Assuming you meant like: how do we determine the formula for a completely new discovered acid.
Well, we can do the whole qualitative suite to give us an idea of what is in that product and some of its properties. If it's inorganic you'll usually want to go put a strong reducing agent to see if you can make any possible metals in that ions to their cation form and then analyze that, or use instrumental means that determine mass percentage, or burn it and analyze the gases, and so on
We can also put that through a mass spectrometer, that will basically ionize and possibly fragment the molecule and separate these fragments giving us a mass / charge value (since the charge is usually
By studying the fragment's mass we can get a good idea of the structure, specially if it's an organic compound.
If we have at least an idea of the molecular formula of the compound we can run the compound through an Infrared analysis or a Nuclear Magnetic Ressonance analysis and so on.
Determining the structure and formula of a new compound is not easy, and it will most likely be a research topic unto itself.
If after all that it wasn't clear how many protons are liberating you can try determining the number of protons by doing a "titration" and seeing how many slumps the curve has, or determine the