Electrical conductivity of metal chlorides Watch

You-know-who
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(Edexcel, May 2010, Unit 1, Question 15.b.ii)

Suggest why aqueous solutions of calcium chloride. CaCl2 (aq) and barium chloride, BaCl2 (aq) of the same molar concentration have different electrical conductivities.

The answer given in the mark scheme is that the two cations have different sizes. Underneath, they also included this in the mark scheme in brackets:

(could select either the calcium ion because it has more water molecules associated with it OR the barium ion because it has more shells of electrons and so larger)

That's the part that I don't get. Calcium ion is smaller, hence is it more soluble in water? But then how do you explain Ba(OH)2 being more soluble than Ca(OH)2? How does the solubility or having more water molecules associated with it relate to electrical conductivity?
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y?-1*-1=1
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for electrical conductivity think about the extent of ionization, therefore, the more the solubility the more the electrical conductivity as there will be more ions available to carry the electrical charge.
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You-know-who
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(Original post by y?-1*-1=1)
for electrical conductivity think about the extent of ionization, therefore, the more the solubility the more the electrical conductivity as there will be more ions available to carry the electrical charge.
Okay. What are the factors which determine solubility? Take this question for example, is calcium chloride more soluble than barium chloride?
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Borek
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Why do you care about solubility when question states "of the same molar concentration"?

Conductivity is a matter of ion mobility, and in general smaller ions are more mobile. This is very handwavy, but should do in this case.

When they refer to water molecules they most likely mean hydration layers.
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You-know-who
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(Original post by Borek)
Why do you care about solubility when question states "of the same molar concentration"?

Conductivity is a matter of ion mobility, and in general smaller ions are more mobile. This is very handwavy, but should do in this case.

When they refer to water molecules they most likely mean hydration layers.
Because I thought it probably had some connection to calcium chloride being more soluble, thus more ions form in water, hence can conduct more electricity... or something like that.

Anyways, so it's basically just: Ba ions are larger, hence less mobile, right? Thank you for clearing this up!
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Borek
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(Original post by You-know-who)
Anyways, so it's basically just: Ba ions are larger, hence less mobile, right?
Yes.

Note: that's only a simplified approximation, as what is really moving is not just a cation, but cation with a hydration layer (or several), so its size is not just that of a cation. To make things more complicated, amount of hydration layers even for cations of the same charge depends on their "naked" radius (smaller cations attract water molecules much stronger).
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