# AP Chemistry Unit 7.11 Introduction to Solubility Equilibria

Introduction to Solubility Equilibria

• For solids dissolving to form aqueous solutions
• Ksp = solubility product constant =
• Ksp value depends on ion concentration → more cations & anions = higher value
• Solids not included in equilibrium expression because the concentration does not change
• Only affected by temp
• Solubility = s = concentration of solid that dissolves
• Solubility has the same units as molarity (moles/liters)
• Solubility can vary with different factors → can speed or slow down the attainment of equilibrium but not its value
• Ex: Speed up by increasing surface area (grinding up the solid or stirring the solution); decrease as with common ion effect

Solubility Product Practice

• Type 1: Given solubility → calculate Ksp value
1. Write out balanced equation & ICE table
• S instead of x → solubility is our s that will plug in
• Type 2: Given Ksp → calculate solubility:
1. Write out balanced equation and ICE table
• Solve for s

Relative Solubilities

• In questions, will be given salts and must be able to decide which one is more soluble
• Ksp will only allow us to compare the solubility of salts that fall apart into the same number of ions
• Bigger Ksp value = more soluble
• Ex: same number of ions so can compare
• Ex: diff number ions so can’t use Ksp to compare
• Will have to solve for solubility (s) and compare those → bigger s value = more soluble

Calculations Involving Precipitation/Will a Precipitate Form?

• Must calculate Q and compare it to Ksp
• Focus on the compound whose Ksp value has been given and write balanced equation for how it breaks up
• Steps:
1. Set up Q using FINAl Molarity (if not given use M1V1 = M2V2)
• Do M1V1 = M2V2 for both solutions → plug in the 2 M2 into Q
• Note: V2 = volume of the two solutions combined
• Q > Ksp → Precipitation
• Q < Ksp → No precipitate
• Q = Ksp → Equilibrium
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