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From what I understand, the solubility is given in terms of molar solubility (or $\pu{K_{sp}}$), from which it can be easily calculated). Indeed, the saturation point represents the maximum amount of a slightly soluble substance in an aqueous solution that solubilizes (dissociates). Therefore, in extremely small amounts even very sparingly soluble salts with extremely small molar solubilities (and $\pu{K_{sp}'}$) can be completely ionized. For instance, $\ce{HgS}$ is one of the least soluble inorganic substances with a molar solubility of about $\ce{10^{-27}}$. Now, if I put up to $\ce{10^{-27}}$ moles of HgS in 1L of water, will it be completely dissolved into $\ce{Hg^{2+}}$ and $\ce{S^{2-}}$?

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    $\begingroup$ HgCl2 is reportedly in large extent covalent. Some salts forms ionic pairs, like MgSO4 in sea water. Other may form charged or neutral complexes. $\endgroup$
    – Poutnik
    Jul 29 at 5:18
  • $\begingroup$ This is an equilibrium process and so to calculate the extent of reaction you can use the usual methods starting with different initial amounts. $\endgroup$
    – porphyrin
    Jul 29 at 7:16
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    $\begingroup$ A solubility of $\ce{10^{-27}}$ M corresponds to $1$ atom $\ce{Hg}$ and $1$ atom $\ce{S}$ in $10^4$ liters, or in $10$ metric tons. This is practically zero atom per litre. This amount is not detectable. $\endgroup$
    – Maurice
    Jul 29 at 8:22
  • $\begingroup$ You cannot put less than $\pu{\frac{1}{N_\mathrm{A}} mol}$ of anything anywhere, as it would be less than 1 object. $\endgroup$
    – Poutnik
    Jul 29 at 10:31
  • $\begingroup$ Yes. Ksp describes a saturated solution. If from dissolution of a salt stoichiometric quantities of cation & anion will be dissolved ("in solution') $\endgroup$ Jul 29 at 12:28

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We should not expect salts with multiply charged ions to be completely dissociated in aqueous solution except, perhaps, at very low concentrations. The comments refer to magnesium sulfate, a technical reference for which is Sebastiani et al.[1]. Note that in this case the paired ions are not in direct contact but separated by solvent molecules, so it's an "outer-sphere" association.

Reference

Federico Sebastiani, Ana Vila Verde, Matthias Heyden, Gerhard Schwaab, and Martina Havenith (2020). "Cooperativity and ion pairing in magnesium sulfate aqueous solutions from the dilute regime to the solubility limit". Phys. Chem. Chem. Phys., 22, 12140-12153.

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