pH of a Calcium Hydroxide vs Magnesium Hydroxide solution [closed]








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> 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. 
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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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