that's right, pubchem doesn't take into account that the salt is diprotic, it just adds the masses of the two compounds.
Acids like fumaric, or sulfuric, are diprotic, meaning they have two acidic sites that can donate 2 protons, so when salted correctly, each molecule of fumarate (COO-) will be associated with two molecules of DMT (NH3+) So when calculating the mass, you calculate for that mole ratio. 2mol DMT to 1mol fumarate.
It's possible to form the monoprotonated salts, these are usually fixed with the prefix bi- like bisulfate, or bicarbonate, so with fumaric, bifumarate. But since these salts are acidic, they usually do not store well, are hydroscopic, do not form crystals easily, and can degrade. This is also part of why its important to add FASA/FASI dropwise with lots of stirring, rather than dumping it in all at once.
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