Determining number of dissociated moles from concentration?

[H Smith 94]

Is there a formula to theoretically determine the number of moles of each ion present when a particular concentration of salt is dissociated in water?

I remember from A-level (high school) chemistry that concepts such as the dissociation constant exist but I'm struggling to apply them to concentration.

I am currently looking at the molar conductivity.

 

[Borek]

Single formula? No. Theory that allows calculation of equilibrium concentrations? Yes.

Or do you mean just a simple case with something that fully dissociates? Then it is not even a formula, it just a direct application of the concentration definition, c=n/V.

 

[H Smith 94]

Hi Borek, thank you for your quick response!

Single formula? No. Theory that allows calculation of equilibrium concentrations? Yes.

That sounds interesting and potentially useful. I'll look it up. Could you shed any light on anything you think may be useful?

Or do you mean just a simple case with something that fully dissociates? Then it is not even a formula, it just a direct application of the concentration definition, c=n/V.

So could I assume that NaCl dissociates perfectly in water, resulting in equal concentrations of Na⁺ and Cl^-? Because that would make everything so much nicer.

 

[Borek]

That sounds interesting and potentially useful. I'll look it up. Could you shed any light on anything you think may be useful?

Check these pages:

http://www.chembuddy.com/?left=pH-calculation&right=general-pH-calculation

http://www.chembuddy.com/?left=pH-calculation&right=toc

While this is about pH calculation, the ideas behind are the same no matter what ions/species are involved.

So could I assume that NaCl dissociates perfectly in water, resulting in equal concentrations of Na+ and Cl-? Because that would make everything so much nicer.

For NaCl (and HS level) you can make such an assumption. At the undergraduate level you will be told it is not that simple, at the graduate level you will find details are a nightmare.

 

[DrDu]

Maybe Ostwald's law of dilution is what you are looking for:
https://en.wikipedia.org/wiki/Law_of_dilution

 

[H Smith 94]

Hi everyone, thank you for your help so far!

Does anyone know how the electrical conductivity $\sigma$ (conversely, resistivity $\rho$) depends on salinity $S$ -- or more specifically, on the movement of dissolved ions in an electric field? I know they're in some way proportional but cannot find how!

 

[Borek]

Google Kohlrausch law.

 

[H Smith 94]

Google Kohlrausch law.

Thank you! Yes, this is a very useful model. I've tried using it previously but found it limited by its semi-empirical nature.

I actually posted a question on this forum about it (which I completely forgot about until just now, oops!) If you wouldn't mind, perhaps you could look at my calculations on that post to see if the assumptions made make chemical sense.

I have no idea why I posted that in the Atomic, Solid State & Comp. forum, no wonder it got no answer!