What should I assume about dissociation here...

[going2breakdown]

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Here I have CaF2 added in and NaF in solution. According to my book, I can assume that NaF is soluble because all Group I element containing ionic compounds will dissociate. However, it does not say whether or not group II ionic compounds will always dissociate. What should I assume. i.e. what dissociates with respect to group II salts?

 

[LoLCareerGoals]

I would also assume complete dissociation.

 

[MedPR]

.

 

[Hanguk]

I think I read in EK that Ca2+, Ba2+, and Mg2+ are all soluble cations.

 

[LoLCareerGoals]

i think i read in ek that ca2+, ba2+, and sr2+ are all soluble cations.

fyp.

 

[Hanguk]

Thanks 😀

+ I had to look up what fyp meant

 

[sports]

I think this question is related to the concept of common ion effect (if it is the case, below if the explanation):

CaF2 is added to NaF - it doesn't matter if there is complete or partial dissociation, the concentration of F- will increase regardless, forcing the reaction to shift towards the reactants side based on the Le Chatelier's Principle.

Concentration of CaF2 and NaF will increase
Concentration of F- will also increase since it is being added
Concentration of Na+ and Ca2+ will decrease

However, I think that group I and group II metals attached to halogen gases will always dissociate completely (someone correct me if I am wrong on this) but based on all the examples I can think of, it is complete dissociation.

 

[gettheleadout]

sports has the right idea here. Your first thought upon reading the problem (or the OP) should be common ion effect mediated by F- upon dissociation.

That said, EK lists Ca2+ as soluble only when paired with S- or OH-. Wikipedia confirms that CaF2 is very marginally soluble in water.

 

[LoLCareerGoals]

Yeah I just looked up CaF2 in an appendix and unlike all other halogen salts it has a Ksp = 10^-11ish, so it is not very soluble. I sort of assumed it would behave like all other salts, but I guess it makes sense since HF is the only weak halogen acid. If that's the case solubility will be affected by presence of existing F-s (common ion effect).
Yet again I learn something on SDN that my HWs haven't stressed. 🙂