Correct solubility product value

[ArjenB]

Hi,

For school we are currently working with heterogeneous equilibria. I am given a salt that will be solved in water and I have to calculate the concentrations of the ions. I have to use the solubility product for this. In the Netherlands we are provided with a reference book that has all common values and constants of physics and chemistry in it. I have both the old 5th edition and newest 6th edition of this reference book, which is the only one we are allowed to use in exams. So I look up the Ksp for calcium fluoride. The 6th edition tells me 3,5 * 10^-11. After having calculated the concenstrations, it turns out my answers were wrong. In my answers booklet a value of 1,5*10^-11 was used for the Ksp instead of 3,5. I knew my chemistry book is old, so I checked on the 5th edition for the chemistry references. And here it told me a value of 1,5*10^-11... Okay, obviously my old chemistry book was matched up with the older reference book. But still, why was it changed? I got confused and I checked on the internet and I am seeing different values of the salt everywhere... 3,9*10^-11, 5,9*10^-9... I mean, how can a constant of physics and chemistry not be clear? Am I missing something here? I checked for other salts and I am finding very different Ksp's for them too. Most match up between my 5th and 6th edition reference books and the internet as well, but some of them are completely different everywhere!

 

[Borek]

First, these constants are results of measurements, so they are never perfect. Second, when they were measured, they were often measured for different conditions - different pH, different ionic strength and so on. In a perfect world tables should contain all these information together with the constant, then it is clear why the values are different. Sadly, this world is far from being perfect and people aggregating solubility/dissociation constants often make lists ignoring all the additional information, thus making the list full of inaccuracies. If two lists give different values it often means these are taken from different sources and were measured in different conditions.

On top of that are all possible typos added during tables preparation.

 

[DrDu]

Second, when they were measured, they were often measured for different conditions - different pH, different ionic strength and so on. In a perfect world tables should contain all these information together with the constant, then it is clear why the values are different.

While the tables usually don't contain this information, they usually contain at least a reference to the paper where the measurement was published and from which this information can be extracted.

 

[Borek]

While the tables usually don't contain this information, they usually contain at least a reference to the paper where the measurement was published and from which this information can be extracted.

Sadly, that's rarely case with the supplemental material in textbooks and problem books - these just list values to use when solving problems, without bothering to name the source.

Sure, serious collections of the constants (like the one I am using, Handbook of Chemical Equilibria in Analytical Chemistry, Kotrly and Sucha, Ellis Horwood Ltd. 1985), list not only sources, but even particular conditions in which every constant was determined.