Ka and Kb are the equilibrium constants related to weak acids and weak bases respectively.
Concept #1: The magnitudes of Ka and Kb determine the strengths of acids and bases.
Example #1: Identify the strongest acid from the following list of weak acids based on their Ka values. Assume temp is 25ºC.
a) HCNKa = 4.9 x 10-10
b) H2O Ka = 1.0 x 10-14
c) HNO2 Ka = 4.6 x 10-4
d) HC3H5O3 Ka = 1.4 x 10-4
Practice: Hypobromous acid (Ka = 2.8 × 10−9) and hydrocyanic acid (Ka = 4.9 × 10−10) are both weak acids. Determine if reactants or products are favored in the following reaction.
HBrO (aq) + CN− (aq) ⇌ BrO− (aq) + HCN (aq)
a) reactants b) products c) both directions are favored equally d) neither direction is favored
Practice: Identify a Bronsted-Lowry acid with weakest conjugate base.
a) H3BO3 Ka = 5.4 × 10−10
b) HF Ka = 3.5 × 10−4
c) HNO2 Ka = 4.6 × 10−4
d) HClO Ka = 2.9 × 10−8
Concept #2: The ionization constant of water connects Ka and Kb.
Example #2: Aspirin, also known as acetylsalicylic acid (Ka = 3.3 x 10-4), is a medication used to reduce pain, fever, and inflammation. Calculate the Kb of acetylsalicylate (C9H7O4-).
Practice: Identify which of the compounds is the strongest species.
a) Iodic acid pKa = 0.80 b) Acetic acid pKb = 9.24 c) Formic acid pKa = 3.75 d) Ammonium pKb = 4.75
Practice: Determine the pKa given the Kb of the following bases:
i) NH3 Kb = 1.76 × 10−5 ; NH4+ pKa = ____________
ii) C6H5NH2 Kb = 3.9 × 10−10 ; C6H5NH3+ pKa = ____________
