Solution Chemistry

Ions in solution

  • Anion, cation (common names, formulas and charges for familiar ions; e.g., NH4+, ammonium; PO43-, phosphate; SO42-, sulfate)
    Common nameFormula
    Halide, hypohalide, etcX-, XO-, etc
    Hydrogen Carbonate (Bicarbonate)HCO3-
    Hydrogen Sulfate (Bisulfate)HSO4-
    Hydrogen PhosphateHPO42-
    Dihydrogen PhosphateH2PO4-
  • Hydration, the hydronium ion
    • hydration / solvation
    • Another name for hydration is solvation.
    • Hydration is where water forms a shell around ions in solution.
    • The oxygen atom on water is partially negative, so it surrounds cations.
    • The hydrogen atoms on water is partially positive, so they surround anions.
    • Hydronium ion = H3O+
    • H+ never exist as a proton in water, it always exists as the hydronium ion.


  • Units of concentration (e.g., molarity)
    • Molarity = M = mol solute/L solution
    • Molality = m = mol solute/kg solvent
    • Normality = N = Molarity of the species that matter.
      • 1 M HCl = 1 N HCl
      • 1 M H2SO4 = 2 N H2SO4
      • 1 M H3PO4 = 3 N H3PO4
    • x % = x g / 100 g = x g / 100 mL
    • x ppm = x parts per million = x mg / kg = x mg / L
  • Solubility product constant, the equilibrium expression
    • Solubility product constant = Ksp
    • AgCl (s) ↔ Ag+ (aq) + Cl- (aq)
    • Ksp for AgCl = [Ag+][Cl-]
    • Ag2SO4 (s) ↔ 2Ag+ (aq) + SO42- (aq)
    • Ksp for Ag2SO4 = [Ag+]2[SO42-]
    • Ksp values are found in a table:
      • Ksp for AgCl = 1.8 x 10-10
      • Ksp for Ag2SO4 = 1.2 x 10-5
    • Ksp is simply Keq for dissolutions.
    • The higher the Ksp, the more the reaction products dominate in a saturated solution (at equilibrium).
    • What is the solubility of MX2 if given Ksp?
      1. MX2 ↔ M2+ + 2X-
      2. Ksp = [M2+][X-]2
      3. Ksp = [M2+][2M2+]2 (because for every M2+, there's two times as much X-)
      4. Ksp = 4[M2+]3
      5. Solve for [M2+]. Solubility is the same thing as [M2+] because you used Q = Ksp for a saturated solution.
      6. If you solved for [X-] instead, divide your results by 2.
      7. If you were given solubility and asked to solve Ksp, then know that solubility = [M2+] = [X-]/2
  • Common-ion effect, its use in laboratory separations
    • The common-ion effect is simply Le Chatelier's principle applied to Ksp reactions.
    • AgCl (s) ↔ Ag+ (aq) + Cl- (aq)
    • The common-ion effect says that if you add Cl- to the solution above, then less AgCl would dissolve.
    • For example, if you add NaCl to a saturated solution of AgCl, then some AgCl will crash out of solution.
    • Another example: more AgCl can dissolve in pure water than in water containing Cl- ions.
    • In laboratory separations, you can use the common ion effect to selectively crashing out one component in a mixture.
      • For example, if you want to separate AgCl from a mixture of AgCl and Ag2SO4, then you can do so by adding NaCl. This will selectively crash out AgCl by the common ion effect (Cl- being the common ion).
  • Complex ion formation
    • Metal+ + Lewis base: → Complex ion
    • M+ + L → M-Ln+
    • The Lewis base can be charged or uncharged.
    • The Keq for this reaction is called Kf, or the formation constant.
  • Complex ions and solubility
    • The "complex ion effect" is the opposite of the common ion effect.
    • AgCl (s) ↔ Ag+ (aq) + Cl- (aq); M+ + Cl- ↔ M-Cln complex ion.
    • When complex ion forms, the Cl- ion is taken out, so more of the AgCl will dissolve.
    • Alternatively: AgCl (s) ↔ Ag+ (aq) + Cl- (aq); NH3 + Ag+ ↔ Ag-(NH3)n complex ion.
    • Here, the complex ion formation takes out Ag+, again causing more AgCl to dissolve.
  • Solubility and pH
    • Acids are more soluble in bases.
      • HA → H+ + A-
      • Putting the above in a base will take out the H+, thus, more HA will dissolve according to Le Chatelier's principle.
    • Bases are more soluble in acids.
      • B + H+ → BH+
      • Putting the above in an acid will add more H+, and thus, drive more B to dissolve according to Le Chatelier's principle.