In an AQA 2017 Chemistry paper, a $0.0131 \pu{mol}\cdot\pu{dm^-3}$ saturated solution of calcium hydroxide is prepared and then the question asks to predict the pH of a similar solution of magnesium hydroxide which is stirred until no more solid dissolved.

The answer (stating that $\ce{Mg(OH)2}$ will be less alkaline) only references the comparative solubilities of the two but says to ignore any reference to concentration.

My initial thought was that 'concentration' of the hydroxide ions is important in getting to this result as $\ce{Mg(OH)2}$ is less soluble and therefore since less of the solid will dissolve than in the case of $\ce{Ca(OH)2}$, the actual concentration of the hydroxide ions could be say 0.029 (for example and assuming complete dissociation in both solutions) and therefore the solution is more acidic. Is this logic wrong even though I would get to the same result as the mark scheme?


This is a simplistic question which doesn't require a great deal of analysis. When ionic salts dissolve the typical assumption is that they completely ionize. So:

$$\ce{Ca(OH)2 -> Ca^2+ + 2OH-}$$ $$\ce{Mg(OH)2 -> Mg^2+ + 2OH-}$$

The assumption is that the calcium hydroxide will dissolve completely. However the magnesium hydroxide is less soluble and will dissolve incompletely. Hence the calcium solution will be more basic, and have a higher pH.

Now if the solutions are both 0.0131 molar, then the maximum hydroxide concentration can only be twice that or 0.0262 molar, not 0.029 molar.

I'm not sure what sort other data, or data lookup you could do with this question. It seems you just supposed to know that magnesium hydroxide is less soluble, and that a 0.0131 molar solution would exceed the solubility of magnesium hydroxide.

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  • $\begingroup$ Hi MaxW, thanks for your answer, essentially I'm asking is will the concentration of the hydroxide ions that are actually in solution for the Mg case be less than that of Calcium's (given they both completely ionise)? $\endgroup$ – user77287 Apr 22 '19 at 16:19
  • $\begingroup$ According to Wikipedia at 25 °C the solubility of magnesium hydroxide is 0.0064 g/L or 0.00011 molar. So if the concentration was less than that, then yes both the magnesium hydroxide and the calcium hydroxide solution would have the same pH. // Just for chuckles, Wikipedia states that at 20 °C the solubility of calcium hydroxide is 1.73 g/L or 0.023 molar. $\endgroup$ – MaxW Apr 22 '19 at 16:31
  • $\begingroup$ @user77287 - I changed the wording of my answer a bit to note that complete ionization happens when the salt dissolves. So if it dissolves incompletely then the molarity of the solution is obviously less than the moles per liter added. $\endgroup$ – MaxW Apr 22 '19 at 16:34
  • $\begingroup$ Thanks very much for the clarification, I was only doubting the molarity side as the mark scheme said to ignore any reference to concentration/molarity but I guess it's an obvious follow-on $\endgroup$ – user77287 Apr 22 '19 at 16:42
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    $\begingroup$ @AdnanAL-Amleh - If the $\ce{Mg(OH)2}$ doesn't dissolve, it stays solid $\ce{Mg(OH)2}$. // Yes I suppose some Mg would form a complex like $\ce{Mg(OH)^+}$ in high concentrations of $\ce{OH^-}$, but it would have a very small concentration relative to $\ce{Mg^{2+}}$ when $\ce{Mg(OH)2}$ dissolves. $\endgroup$ – MaxW Jul 12 '19 at 6:29

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