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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