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I'm able to find a lot of information on acids and bases, as well as strong-strong, weak-strong, and strong-weak salts. However all the introductory materials on electrolytes just stop there and say "we won't bother to cover weak-weak salts here". I have also heard blunt statements that all salts are strong electrolytes, but never with satisfactory explanation or even some weak-weak examples. So are there any salts that are weak electrolytes?

My interest in the subject is mainly directed at finding the van 't Hoff factor, for osmotic effects and tonicity.

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  • $\begingroup$ Weak-weak salts are typically hydrolyzed all the way to the end, so their solutions are not a thing at all. If they don't do so, they are probably not all that weak. $\endgroup$ – Ivan Neretin Apr 19 at 17:41
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    $\begingroup$ From wiki, ammonium acetate has a solubility of 143 g per 100 mL of water at 20 deg C. Lots of ions in that solution, that can move in response to a potential difference impressed across a separated pair of inserted electrodes. Seems like a strong electrolyte to me. $\endgroup$ – Ed V Apr 19 at 17:53
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When a weak acid such as acetic acid is added to pure water, it does not ionize much, but sufficiently to make the solution acidic. On the other hand, when small amounts of weak acids are added to buffered solution with a pH near neutral, acidic acid dissociates almost quantitatively. So the van't Hoff factor depends on the pH.

The same goes for weak bases used as ammonia - in combination with pure water, you get a basic solution and very little ammonium, but when dissolved in a solution maintaining a neutral pH, ammonia forms the ammonium ion almost quantitatively.

I have also heard blunt statements that all salts are strong electrolytes, but never with satisfactory explanation or even some weak-weak examples.

If we combine ammonia and acetic acid in aqueous solution and remove the water, we get the salt ammonium acetate. If we dissolve ammonium acetate in pure water, the pH will be roughly neutral, and both ions will remain ions. Therefor, this is a strong electrolyte solution.

So are there any salts that are weak electrolytes?

In the pure salt, the components are ions by definition. Water is very good at dissolving ions, so the dissolved ions stay ions. To get a salt that is also a weak electrolyte, you would need an uncharged acid and an uncharged base that would transfer a proton upon forming a salt, but not while in aqueous solution. I am not aware of any such system.

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  • $\begingroup$ So even with fairly concentrated solutions of weak-weak salts, the dissolved salt will have complete [or nearly] dissociation of the ions? Just making sure I understand. Would this also mean that precipitation is inevitable when the ions re-associate? It is just difficult for me to see that HA or BOH can readily exist in solution along side -A or +B but BA cannot exist in a significant fraction of the solute. Probably one of those things i just need to memorize for now and will understand later. (ammonium hydrogen phosphates would be an interesting example, what with the protons and all.) $\endgroup$ – Max Power Apr 20 at 0:05
  • $\begingroup$ Yes, when ionic compounds dissolve, each cation is solvated by the solvent, completely separated from solvated anions. And yes, the only time the cations re-associate with the anions is when the ionic compound precipitates. H-A is a covalent bond, so that can exist in solution. BA is an ionic bond, not observed in aqueous solution. The notation BOH is incorrect. For example, NH3 is the base, and NH4+ is the conjugate acid (the ionic form), so there is no involvement of hydroxide ions (those occur in strong bases, e.g. NaOH). $\endgroup$ – Karsten Theis Apr 20 at 1:33
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    $\begingroup$ @KarstenTheis Ammonium cyanide is an ionic compound that is very soluble in water. If you make 0.1 M solution, the slightly majority species are aqueous ammonia and aqueous HCN, while ammonium ion and cyanide ion are slightly minority species. But it is still a strong electrolyte and the solution’s electrical conductance is a separate matter: a sufficiently dilute solution of any strong electrolyte is a (like pure water) poor electrical conductor. I think the strong/weak electrolyte notion is separate from the more nuanced electrical conductivity matter. Am I off-track in this? $\endgroup$ – Ed V Apr 20 at 18:33
  • $\begingroup$ @EdV, I think the best figure of merit for judging electrical conductivity of a salt is its conductivity at infinite dilution. $\endgroup$ – M. Farooq Apr 20 at 18:39
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    $\begingroup$ @M.Farooq I already upvoted Karsten’s answer and will upvote yours right after this. Both answers are good contributions. $\endgroup$ – Ed V Apr 20 at 19:15
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Good question, the surprising thing is that you cannot predict conductivity easily. Ammonium acetate, which is a salt of weak acid and weak base, is a comparable conductor as a salt formed by strong acid and strong base (e.g. NaCl). This is a really old table but look at the cases of a) Sodium chloride: salt of strong acid and strong base b) Sodium acetate: salt of strong base and weak acid c) Ammonium chloride: salt of weak base and strong acid d) Ammonium acetate: weak acid weak base

Not all salts are strong conductors, search the data for organic salts such as potassium citrate - a weaker conductor than many salts.

If you study the table carefully, it shows the strength of acid or base does not matter but what matters is the identity of the ions carrying the current. Also, the concentration of the salt also matters.

Table

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  • $\begingroup$ Certainly, in the citrate case, arguments regarding the softness of the ion and size / stearic & kinetic hindrance are a relevant. I'd say that given full solubility, the size and "softness" are important factors for conductivity - more so than the strength of the acids / bases (although that hints at solubility) $\endgroup$ – Stian Yttervik Apr 20 at 8:38
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    $\begingroup$ Karsten, this is the equivalent conductance at infinite dilution. So basically,you would plot the conductance with non-zero concentrations and then extrapolate the concentration to zero. The y-value gives the maximum equivalent conductance. Recall the Kohlrausch's laws. $\endgroup$ – M. Farooq Apr 20 at 17:38
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    $\begingroup$ Equivalent conductance is the same as molar conductance, except for the fact the concentration is given in normality = gram equivalents/ Volume in liter. Conventionally, the conductivity is still measured and quoted in gram equivalents. $\endgroup$ – M. Farooq Apr 20 at 17:40
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    $\begingroup$ @M.Farooq Please see my comment under Karsten’ answer. I think the strong and weak labels have not aged so well over the decades: edge cases are not all that rare, e.g., calcium hydroxide. I consider it to be a type of strong base, i.e., a strong electrolyte that is also base, but it is also only sparingly soluble. But not everyone would agree with this. $\endgroup$ – Ed V Apr 20 at 18:37
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    $\begingroup$ @EdV, I don't see any problem with your statement. I associate strong and weak with pKa or pKb values. However, conductivity, and I suggest conductivity at infinite dilution should be discussed. As you know conductivity is a function of concentration. The conductivity at infinite dilute gets rid of all solubility problems. This is the true reflection of an ion's properties in water. $\endgroup$ – M. Farooq Apr 20 at 18:42

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