How to Convert pKa to Ka

Basic Equations

• 1

  The generic Bronsted-Lowry equation for acids and bases is:
  HA + H2O <==> H3O+ + A-.
  In this equation, HA is the acid, which donates a hydrogen to H20, which is the base. On the opposite side of the equation, H3O+ is the conjugate acid and A- is the conjugate base.

• 2

  Divide the concentration of the products by the concentration of the reactants to get Ka:
  [H3O+][A-]/[HA][H2O]=Ka
  Since H20 is a constant, it can be removed from the equation:
  Ka=[H3O+][A-]/[HA]

• 3

  Take the negative log of Ka to get pKa:
  pKa = -log10(Ka)

Sample Problem

• 4

  Find the Ka for the equation: H20 + HCl <==> H30+ + Cl- if the concentrations of H30+ and Cl- are both 0.1 Molars and the concentration of HCl is 10^-8.
  Ka= [0.1][0.1]/10^-8
  Ka=10^7

• 5

  Convert Ka to pKa.
  pKa = -log(10^7)
  pKa = -7

• 6

  Reverse the equation to convert pKa to Ka:
  Ka=antilog(-pKa)
  Ka=antilog(-7)=10^7
  (ref 4)

• 7

  Evaluate the answer. pKa values range from -10 to 50. High pKas are strong bases, while low pKas -- such as HCl -- are strong acids. Low pKa values correspond to large Ka values: In this case, 10^7.