# Calculating molar solubility of calcium hydroxide from solubility product

On the Wikipedia page for calcium hydroxide, the stated solubility product for $$\ce{Ca(OH)2}$$ is $$K_\text{sp} = \pu{5.5 * 10^{-6} M^3}$$. It doesn't stipulate the temperature, but I have a chemistry book that says the exact same value for $$\pu{25 ^\circ C}$$.

However, when I calculate the solubility of $$\ce{Ca(OH)2}$$ in water using this value ($$\pu{5.5 * 10^-6 M^3}$$), I get a result of $$\ce{0.082 g/100ml}$$.

However, according to solubility tables on Wikipedia, calcium hydroxide is only this soluble at around $$\pu{85 ^\circ C}$$. How can this apparent contradiction be explained?

• I think this is very curious, but it seems you answered the question yourself already in chemistry.stackexchange.com/questions/104055/… … . Personally I'd trust the National Lime Association before the wikipedia, after all their money is getting lime right. – Night Writer Nov 8 '18 at 19:58
• But according to the National Lime Association, it still doesn't make any sense. – Hilmar Nov 8 '18 at 20:06
• The only way to sort discrepancies like this is to ignore any data that doesn't reference an original source. In others words you looking for data from a scientific report, not just a data table. Now with the sources you have to look at the experimental errors and techniques used to decide which data is more reliable. You then somehow have to merge all the data into the "accepted values." – MaxW Nov 8 '18 at 22:17
• PS - Kudos for noting the discrepancy!! – MaxW Nov 8 '18 at 22:18

A cursory search suggests the solubilities in g/100 mL on the wikipedia are closer to a consensus, for what it's worth:

These are reasonably consistent with the values in

https://en.wikipedia.org/wiki/Solubility_table#C

but not with the $$K_{sp}=5.5 \times 10^{-6} M^3$$ on the wiki page.

Skoog, West and Holler's Analytical Chemistry textbook (7th Edition) reports $$K_{sp} = 6.5 \times 10^{-6} M^3$$ at 25$$^oC$$. They provide a ref:

most of the data were taken from A.E. Martell and R. M. Smith, Critical Stability Constants, Vol. 3-6.

Note the apparent inconsistency recurs (I called this discrepancy an "error" before but am no longer so sure):

It reminds you that you should generally reference your data sources if you want to be helpful to fellow researchers.

The value for $$K_{sp}$$ in the wikipedia (which is not referenced as far as I could tell) and in a highly respected textbook which references its source is smaller by roughly an order of magnitude compared to values derived using standard equations from reported solubilities in a number of other sources noted above. This inconsistency may be because some of the reported data is for hydrates with higher Mw. Not sure how you could explain such a discrepancy otherwise. If the $$K_{sp}$$ were much smaller you could argue that the dissociation constant of water affected the solubility.

My current guess is that there is a reasonable explanation for the discrepancy (ie it has a chemical or physical explanation and is not due to human error). It is unfortunate that the solution is not more obvious given that $$\ce{Ca(OH)2}$$ is a recurrent example in problems (textbook calculations or experiments) teaching solubility to undergraduates.

Solubility of calcium hydroxide determined by titration

• Even if it was off by an order of magnitude it still wouldn't make any sense if you try to calculate it. – Hilmar Nov 8 '18 at 21:53
• Yea there has to be an explanation. I just wish someone knew it:/ Thanks for the help anyway, I appreciate it. – Hilmar Nov 9 '18 at 11:51
• I think you search a little further you may stumble on the explanation. I'd try your teacher. – Night Writer Nov 9 '18 at 18:55
• I’ll try to search a little more, and my teacher doesn’t know the explanation – Hilmar Nov 9 '18 at 19:01