Question concerning solubility equilibriums for salts

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In summary, when a salt with low solubility is mixed into water, the equilibrium constant for the solubility is calculated using the concentrations of the ions present, rather than the concentration of the undissociated substance. This is because even though the salt may not be fully dissolved, the ions from the salt can still be present in the solution. This can be compared to a large crystal of the salt being in contact with the water, where the concentration of the undissociated substance is zero, but the ions can still be leached into the solution. Therefore, the concentration of the undissociated substance, [AB], is considered to be equal to 1 in these cases.
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Nikitin
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Hi. Let's say we have a salt AB with a very low Ksp. A bunch of it gets thrown into the water and mixed.

So:

K=[A+][B-]/[AB].

Ksp=[A+][B-]

Why is it OK to remove [AB], then? ie why does [AB] equal 1? I thought [AB] was concentration of the substance in the water, but it is the substance's activity in the water, according to wikipedia?

Please explain this so a high-schooler can understand.
 
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[AB] would be concentration of the substance, but the substance is not dissolved.

Imagine you have a single large crystal of a weakly soluble salt, that is a wall of the vessel in which you are dissolving the substance. Concentration of the undissociated substance in the solution is zero, however, it is in the contact with water, so the ions can can be leached into the solution. When you have smaller crystals situation is identical, they form a separate phase.
 

1. What is solubility equilibrium for salts?

Solubility equilibrium for salts refers to the dynamic balance between the dissolved and undissolved forms of a salt in a solution. It occurs when the rate of dissolution of the salt is equal to the rate of precipitation, resulting in a constant concentration of dissolved ions.

2. How is solubility equilibrium affected by temperature?

In general, solubility increases with increasing temperature for most salts. This is because higher temperatures provide more energy for the solvent molecules to break apart the ionic bonds between the salt molecules, resulting in more dissolved ions. However, there are some exceptions to this rule, such as in the case of endothermic reactions where solubility decreases with increasing temperature.

3. Can solubility equilibrium be altered by changing the pH of the solution?

Yes, the pH of a solution can greatly affect the solubility of a salt. In some cases, changing the pH of a solution can shift the equilibrium towards the dissolved or undissolved form of the salt, resulting in changes in solubility. This is because the pH can affect the ionic strength of the solution, which can influence the strength of the ionic interactions between the salt molecules.

4. What factors can influence the solubility equilibrium of a salt?

Aside from temperature and pH, other factors that can affect solubility equilibrium for salts include pressure, the presence of other solutes, and the nature of the solvent. For example, increasing pressure can lead to an increase in solubility, while the presence of other solutes can either increase or decrease solubility depending on their interactions with the salt molecules.

5. How is solubility equilibrium measured?

Solubility equilibrium can be measured experimentally by determining the concentration of the dissolved ions in a solution at different temperatures or pH levels and then plotting this data on a solubility curve. The point at which the curve intersects with the line representing the undissolved form of the salt represents the equilibrium point, where the rate of dissolution equals the rate of precipitation.

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