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In a system, if we have $\pu{18 g}$ of $\ce{H2O}$ ($M_\mathrm r = 18$), we can say we have a mole of water molecules or $6 \times 10^{23}$ molecules. But in another system, if we have $\pu{342 g}$ of $\ce{Al2(SO4)3}$ ($M_\mathrm r = 342$), what is the best way to say that there is one mole of "aluminum sulfates" or $6 \times 10^{23}$ "aluminum sulfates"?

Since it is a ionic compound, we can't say molecules, but I can't find an analogous form for ionic compounds. I thought about saying "a mole of aluminum sulfate ionic aggregates", but it still doesn't too good, since I've never seen a definition of ionic aggregate as a unit of the ionic compound formula. It only tells the way the ions organize themselves: as an aggregate.

I know it is a silly question, but in terms of language and symbolism, it is important to me as a chemistry teacher.

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  • $\begingroup$ Note that you might need to give amounts of each ion separately anyway, due to common ion effect. So having only one salt with equal parts of cat and an will be a special case. $\endgroup$ – JDługosz Sep 12 '17 at 4:48
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The term you are looking for is formula unit, I think. Wikipedia doesn't really describe it super well, but just to give an example, you could write the sentence

$\pu{58.44 g}$ of $\ce{NaCl}$ contains $6.022 \times 10^{23}$ formula units of sodium chloride

and it would be pretty well understood.

See also: What is the difference between a Chemical Formula and Formula Unit?

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    $\begingroup$ Depending on what is meant by a chemical formula, the terms chemical species, and molecular entity might also be what OP is after. In short, chemical species is a collection of $\ce{NaCl}$ formula units (e.g., as a big solid) whereas molecular entity is one particular unit (molecule, atom, radical, etc). Ion pair is also pertinent in this case. See DOI: 10.1351/pac199466051077 for more details. Feel free to add to your answer if you like. $\endgroup$ – Linear Christmas Sep 11 '17 at 20:13
  • $\begingroup$ Thanks for the link and for the extra information. I can see "molecular entity" being used too, perhaps? For the others, I can't quite construct a good sentence with them. "Ion pair" is generally used to emphasise close association of ions in solution, in my own experience, at least. $\endgroup$ – orthocresol Sep 12 '17 at 6:57
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    $\begingroup$ Not a problem. I believe IUPAC permits using ion pair in similar meanings, the more accurate term for what you describe is tight ion pair. There are also loose ion pairs which are, in turn, categorised by the number of solvent molecules in between. Solvent-shared ion pair refers to there being only one solvent molecule whereas they're solvent-separated if more than one molecule intervenes (IUPAC mentions to be careful with the loose distinction due to ambiguity.). Too many solvent molecules in between, or in the gas phase with sufficient separation the term free ions is used. $\endgroup$ – Linear Christmas Sep 12 '17 at 8:19

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