Typesetting of (formal) addition compounds according to IUPAC using chemmacros

First of all: leave more spaces in the input. The detailed syntax for ch is described in the chemformula manual. Especially stoichiometric factors and formulas need to be surrounded by spaces (that holds for the optional arguments of the arrows as well) even if they're part of a larger formula like in hydrates. This is shown in section 6.1 of the chemformula manual:

The correct syntax for your example is like this:

documentclass[12pt]{article}
usepackage[ngerman]{babel}
usepackage{chemmacros}

begin{document}

ch{CuSO4 ->[ 5 H2O ] CuSO4 * 5 H2O}

end{document}

If you now want to adjust the space surrounding the dot you can set the option adduct-space to a value that suits you. The IUPAC Red Book indeed both has no spaces surrounding the dot and no spaces between compounds and stoichiometric factors which means both adduct-space and stoich-space need to be set to 0pt for getting the same output:

documentclass[12pt]{article}
usepackage[ngerman]{babel}
usepackage{chemmacros}
chemsetup{
  formula = chemformula ,
  chemformula/adduct-space = 0pt ,
  chemformula/stoich-space = 0pt
}

begin{document}

ch{CuSO4 ->[ 5 H2O ] CuSO4 * 5 H2O} 

end{document}

I am not convinced, though, that the Red Book is making a typographical recommendation here, since the chapter only describes the naming of addition compounds. The Green Book on the other hand has a lot of typographic advice. Although it doesn't explicitly mention it it does use spaces to separate compounds from stoichiometric factors which implies to me that the spaces actually do follow the IUPAC recommendation.

---

Please also note that you should leave no space between arrows and their arguments. The typeset result of your input clearly shows this:

ch{CuSO4 -> [][5 H20] CuSO4*5H2O}