# Naming Compounds

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1. Nov 15, 2014

### minimario

How do you name compounds such as Ammonium Aluminium Sulfate?

It should first be (NH4)+ then Al 3+ and SO4 2-, but how do you put then together? How do you know it is $NH_4 AL (SO_4)_2$ instead of, say, $(NH_4)_3 Al (SO_4)_3$?

Thanks!

2. Nov 15, 2014

### Staff: Mentor

I am afraid it is one of the things you have to remember. That is, you don't have to remember every possible salt separately, most mixed salts containing ammonium and Me3+ have NH4Me(SO4)2 formula. But when it comes to hydration waters there are no rules (or more precisely, the rule is "whatever crystal form has the lowest energy" - but calculating crystal energy is so difficult, it is easier to prepare the salt and determine amount of water experimentally).