Alum salt nomenclature

the rule of nomenclature of an alum salt is quite simple and very different from the rule used in other types of salts. THE nomenclature of a salt any is made based on the name of the anion and in the name of the cation. In the case of an alum, we use the following rule:

Alum + de + Cation name +3 + e + Cation name +1

In the nomenclature of alum salt, we do not use the anion name, because they all have the same formulation scheme, that is, we have the presence of two sulfates (SO₄). The first sulfate is linked to the cation +1 (X) and the other is linked to the +3 (Y) cation, respectively in that order, as we can see below:

X₂ONLY₄.Y₂(ONLY₄)₃.24H₂O

Thus, based on the general formula for an alum salt, shown above, we can easily identify the +1 cation and the +3 cation present in the salt below and then provide your name in accordance with the naming rule:

K₂ONLY₄.In₂(ONLY₄)₃.24H₂O

In the formula above, the +1 cation is potassium and the +3 cation is indium. Thus, following the naming rule and writing the name of +3 followed by +1, the name of this alum salt will be:

Indium and Potassium Alum

There are still some particularities about the nomenclature of an alum salt. See them below:

1^O case: If the +3 cation present in the alum is aluminum, it is not necessary to write its name. Thus, only the +1 cation is part of the salt nomenclature. The example below shows this situation:

At₂ONLY₄.Al₂(ONLY₄)₃.24H₂O

Sodium alum

2^O case: If the cations present in the alum formula do not belong to the IA and IIIA families and are not the silver (Ag), we must indicate the charge of each of them in the name by means of a number Roman. Note the example below:

Au₂ONLY₄.Cr₂(ONLY₄)₃.24H₂O

The +1 cation is gold (Au), which belongs to the IB family, and the +3 cation is chromium (Cr), which belongs to the VIB family. Therefore, for Au, we use the number I and, for Cr, we use the number III:

Chromium III Alum and Gold I

See now other examples:

Example 1: Ag₂ONLY₄.Ni₂(ONLY₄)₃.24H₂O

In the presented salt, we have the silver (Ag) as +1 cation it's the nickel (Ni) how the +3 cation. As nickel belongs to the VIIIB family, we must use the numeral III to indicate its charge. So, the name of this alum is:

Nickel III and Silver Alum

Example 2:Bismuth III Alum and Copper I

When we have the name of the alum, we can build its formula. For this, we must remember that your formula must have two sulfates (SO₄), the first being the +1 cation (always written last in the given name; in this case, copper-Cu) and the second the +3 cation (always written first in the given name; in this case, bismuth-Bi), followed by 24H₂O. Thus, the formula for the salt in question is:

Ass₂ONLY₄.Bi₂(ONLY₄)₃.24H₂O

By Me. Diogo Lopes Dias

Source: Brazil School - https://brasilescola.uol.com.br/quimica/nomenclatura-sal-alumen.htm