Puzzled by aqueous solution terminology

  • Thread starter dgm
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  • #1
dgm
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Main Question or Discussion Point

I'm just beginning to learn general chemistry, and I'm reading my textbook's chapter on aqueous solutions.

My question is, why do we term a solution of an ionic compound (like NaCl) like this:

NaCl(aq)​

instead of:

Na+(aq) + Cl-(aq)​

? Technically, when the sodium chloride is dissolved in water, the ions dissociate, and they are no longer the ionic compound "sodium chloride", they're just sodium and chloride ions floating in the water willy-nilly, right?



And then, what if you dissolved sodium chloride and potassium nitrate into the same water. Would you write,

NaCl(aq) + KNO3(aq)​

? How do you know that it hasn't become:

NaNO3(aq) + KCl(aq)​

?
 

Answers and Replies

  • #2
Borek
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why do we term a solution of an ionic compound (like NaCl) like this:

NaCl(aq)​

instead of:

Na+(aq) + Cl-(aq)​
The first notation suggests identical concentrations of Na+ and Cl-, the second suggests solution that contains both ions, but their concentrations are not necessarily identical. But don't pay too much attention to this - this is a simplified notation, and it is not necessarily precise/unambiguous.

Technically, when the sodium chloride is dissolved in water, the ions dissociate, and they are no longer the ionic compound "sodium chloride", they're just sodium and chloride ions floating in the water willy-nilly, right?
Yes.

And then, what if you dissolved sodium chloride and potassium nitrate into the same water. Would you write,

NaCl(aq) + KNO3(aq)​

? How do you know that it hasn't become:

NaNO3(aq) + KCl(aq)​

?
Technically it is impossible to find out what salts have been dissolved, which is why - especially in the case of natural waters - we give not amounts of salts dissolved, but concentrations of individual ions, this is unambiguous and precise.

If the solution was prepared by dissolving NaCl and KNO3 first notation suggests identical concentrations of Na+ and Cl-, and identical concentrations of K+ and NO3-, the second notation suggests something else. Again, don't treat the notation too religiously, it is often used in ambiguous way or even abused. When it matters, there are better ways of telling what we mean.
 
  • #3
dgm
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Thanks. :)

Another question that's been bugging me lately: is it possible to isolate certain ions, so that they are not part of any compound? Like, would it be possible to manufacture a container of pure chloride gas, as opposed to chlorine gas?
 

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