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Author Topic: Saturation point understanding, salts in water  (Read 2609 times)

Offline Roju

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Saturation point understanding, salts in water
« on: 16/01/2015 17:08:52 »
Please read, and correct me where i am wrong

I have always thought that when you pour NaCl(s) (or any ionic compound) into water, the 45e46989e3704bc2ba0899724acdca5c.gif-atoms separate the Na+ and Cl- ions, dissolving the salt, and the saturation point is when there aren't any free 45e46989e3704bc2ba0899724acdca5c.gif-atoms left to pull any more NaCl(s) apart, and at that point the excess salt remains solid at the bottom of the solution. Today i learned about the Common ion effect and i stumbled upon a problem.

With my understanding of the saturation point i dont fully grasp how if you have a saturated solution of AgCl and add NaCl(s), ignoring the change in volume, you can completely ignore the dissolved Na+-ions when calculating the solubility of AgCl in the new solution. Surely the Na+-ions will effect the solubility of AgCl if they occupy some of the limited 45e46989e3704bc2ba0899724acdca5c.gif-atoms(which separate the salts into individual ions), as they are dissolved in the solution also.
If you only added Cl- ions i am completely onboard with the idea of the concentration of AgCl(s) in the solution going up, becuase there are too many free ions for the limited 45e46989e3704bc2ba0899724acdca5c.gif-atoms to manage and therefore you get precipitation.

Can someone tell me where im going wrong, and try to explain to me how it works?
What determines how soluable a salt is?

Thanks in advance.


 

Offline chiralSPO

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Re: Saturation point understanding, salts in water
« Reply #1 on: 16/01/2015 17:31:30 »
There is usually more than enough water to any any solution to "bind to" the ions. Water is about 55 molar, and there are only a few ionic substances that can be dissolved in concentrations more than 8 molar...

The real issue is the equilibrium between ionic solid and solvated ions--the ions have opposite charges, so they are attracted to one another. We have to account for the attraction of the ions to each other versus their attraction to the water. If it were just about having enough water molecules to surround the ions, how could you explain the difference in solubility between NaCl and AgCl?

As you increase the concentration of ions in solution, you are also decreasing the average distance between the ions, thereby increasing the effect they have on one another.

It easiest thinking about this in terms of solubility equilibrium constant (Ksp): where there is a (temperature-dependent) constant that represents the upper limit of the product of the ions concentrations in solution. For instance:

for AgCl at 25 C, Ksp = [Ag+]x[Cl] = 1.8x1010 mol2/L2

so if a solution has a chloride concentration of 0.01 M, the maximum silver concentration would be 1.8x108

for PbCl2 at 25 C, Ksp = [Pb+2]x[Cl]x[Cl] = [Pb+2]x[Cl]2 = 1.7x105 mol3/L3

as we can see from this equation, even though lead chloride is more soluble than silver chloride, but it is more sensitive to chloride concentration because the chloride concentration is squared.

please look up Ksp online or in a general chemistry textbook for a more detailed explanation.
 

Offline Roju

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Re: Saturation point understanding, salts in water
« Reply #2 on: 16/01/2015 17:47:24 »

The real issue is the equilibrium between ionic solid and solvated ions--the ions have opposite charges, so they are attracted to one another. We have to account for the attraction of the ions to each other versus their attraction to the water. If it were just about having enough water molecules to surround the ions, how could you explain the difference in solubility between NaCl and AgCl?

As you increase the concentration of ions in solution, you are also decreasing the average distance between the ions, thereby increasing the effect they have on one another.


Aha. Good explaination, really cleared up all my questions, I allready know about Ksp, it was just my understanding of what a Saturated solution actually meant that got me confused. Many thanks!
 

The Naked Scientists Forum

Re: Saturation point understanding, salts in water
« Reply #2 on: 16/01/2015 17:47:24 »

 

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