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pH of Weak Bases quiz #1 Flashcards

pH of Weak Bases quiz #1
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  • What is the pH of a 0.280 M solution of (CH3)2NH (dimethylamine), given that the Kb of (CH3)2NH is 5.4 × 10⁻⁴?
    First, set up the base dissociation: (CH3)2NH + H2O ⇌ (CH3)2NH2⁺ + OH⁻. Use an ICE chart to find [OH⁻] at equilibrium. Let x = [OH⁻] formed. Kb = x² / (0.280 - x) ≈ x² / 0.280 (since x is small). Solve: x² = (5.4 × 10⁻⁴)(0.280) = 1.512 × 10⁻⁴; x = √(1.512 × 10⁻⁴) ≈ 0.0123 M. pOH = -log(0.0123) ≈ 1.91. pH = 14 - pOH = 12.09.
  • What is the pH of an aqueous ammonia solution if its pOH is 3.20?
    pH and pOH are related by pH + pOH = 14. Therefore, pH = 14 - 3.20 = 10.80.
  • What is the pH of a 0.145 M solution of (CH3)3N (trimethylamine), given that the Kb of (CH3)3N is 6.4 × 10⁻⁵?
    Set up the base dissociation: (CH3)3N + H2O ⇌ (CH3)3NH⁺ + OH⁻. Use an ICE chart: Kb = x² / (0.145 - x) ≈ x² / 0.145. x² = (6.4 × 10⁻⁵)(0.145) = 9.28 × 10⁻⁶; x = √(9.28 × 10⁻⁶) ≈ 0.00305 M. pOH = -log(0.00305) ≈ 2.52. pH = 14 - pOH = 11.48.
  • What does the acronym ICE stand for when calculating equilibrium concentrations for weak bases?
    ICE stands for Initial, Change, Equilibrium, and is used to organize concentration changes during a reaction.
  • Why do weak bases require the use of an ICE chart in equilibrium calculations?
    Weak bases only partially dissociate, so an ICE chart helps determine the exact equilibrium concentrations of all species involved.
  • What unit is used for concentrations in an ICE chart when working with weak bases?
    Concentrations in an ICE chart are expressed in molarity (M), which is moles per liter.
  • How does the percent ionization of a weak base compare to that of a strong base?
    Weak bases ionize less than 100%, while strong bases ionize completely, or 100%.
  • What formula is used to calculate the percent ionization of a weak base?
    Percent Ionization = ([OH⁻] at equilibrium / initial concentration of weak base) × 100.
  • What does the base ionization constant (Kb) represent in the context of weak bases?
    Kb measures the extent to which a weak base dissociates into ions in aqueous solution.
  • Why is understanding percent ionization important for predicting the behavior of weak bases?
    It helps determine how efficiently a weak base produces ions in solution, which affects its chemical reactivity.