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### Course: MCAT>Unit 9

Lesson 3: Solubility equilibria

# Introduction to solubility equilibria

The solubility product constant, Kₛₚ, is an equilibrium constant that reflects the extent to which an ionic compound dissolves in water. For compounds that dissolve to produce the same number of ions, we can directly compare their Kₛₚ values to determine their relative solubilities. If we know the solubility of a salt, we can use this information to calculate the Kₛₚ value for the compound. Created by Jay.

## Want to join the conversation?

• What would the unit for Ksp be?
• That varies depending on the substances. There is no set units for equilibrium constants and for that reason it is customary to just omit the units, and just let it be understood that all of the concentrations are in units of molarity (M) unless otherwise specified.
• I believe that I am getting mixed up with when to use mol vs M the initial concentration of the ICE table. I remember using mol in some of the problems in the titration lectures. Can you explain?
• where did you write the concentration of reactants?
• At he states that you do not include the concentration of the reactants in the Solubility Equilibrium because it is a pure solid.
• How could PbCl2 be an ionic compound? the difference of electronegativity between the two compounds is not that great.
• If there is any electronegativity difference, then the bond will have some ionic character (even if it's not "fully" ionic). Bonding is not a dichotomy: it is not either covalent or ionic. Bonding happens on a sliding scale between 100% ionic (no electron sharing, electrostatic attraction only) and 100% covalent (electrons are shared exactly equally). PbCl2 falls between these two extremes.
• at 3.50mins you wrote 0.00079mol when the calculation was actually 7.91 mol, why did you do this?? it's already in grams and that's what we want the answer to be in so why make it into kg??
(1 vote)
• The calculator shows 7.9108234448g -4 . You didn't notice the -4,which means multiplying 7.91 with 0.0001. And obviously, when dividing 0.22 by 278.1, you can't get 7.91.
• () I don't understand whY we don't include solids and liquids in the equilibrium expression. If we are saying that it is because their concentrations do not change, isn't this false, as we have seen our solid dissolve into ions, meaning its concentration in the system will decrease?

Even if concentration does not change, is it simply a matter of convention - i.e. we do not include them, since it doesn't matter for comparing Q and K, since we can cancel out the unchanged concentrations from each side?
• Good question btw. So you are correct in that we don't include solids or pure liquids because their concentrations don't change while aqueous and gaseous species do change. This is because liquids and solids are practically incompressible. The molecules in liquids and solids are very close together, with very little room to squeeze any closer. This means their concentrations only depend on their densities which are constant. If part of our solid dissolves into ions, the density of the solid itself does not change. As long as some of the solid remains undissolved, we can disregard it in the equilibrium expression because density remains unchanged. Hope this helps.
• Do we have to put coefficient as exponent for Ka and Kb as well?
• I had the same question since it wasn't mentioned in previous videos!
(1 vote)
• At about the mark he says this is a saturated solution. But isn't a saturated solution one in which all of the solute has completely dissolved into the solvent?
• A saturated solution is a chemical solution containing the maximum concentration of a solute dissolved in the solvent. Additional solute will not dissolve in a saturated solution.
NOT "all of the solute has completely dissolved into the solvent" BUT Additional solute will not dissolve in the solvent (saturated solution) .
• will ksp change if temp change?
• Yes. How a temperature increase affects solubility depends on wheter the dissolving reaction is exo or endothermic. The majority of substances show an exothermic dissolving. In that case, increasing the temperature will decrease solubility.
There's a more mathematical approach, but I like to think about it this way:
A(solid) <---> A(aqueous) + Heat , so, if you increase temperature, the reaction will be able to consume heat energy from the enviroment more easily, what shifts it to the left, decreasing Ksp. On the other hand, if the temperature decreases, the reaction will, almost in a attempt to keep balance, shift to the right and generate heat energy.