What is the pka range for a strong acid?

1 Answer
Feb 16, 2016

It always depends on context. There is no hard and fast rule for a pKa range.

Always ask, "relative to what?" Consider the solvent!

  • #"HI"# is a strong acid relative to water. In water, its pKa is about #-10# vs. #15.7# (water). #K_a ~~ 3.2xx10^9#
  • #"H"_2"SO"_4# is a strong acid relative to water. In water, its pKa is about #-3# vs. #15.7# (water). #K_a ~~ 10^3#
  • #"HF"# is a weak acid relative to water. In water, its pKa is about #3.17# relative to #15.7# (water). #K_a = 7.2xx10^(-4)#

Yet...

  • #"HI"# is a weak acid relative to #"HBr"#. In water, the pKa of #"HI"# is about #-10# vs. #-9# (#"HBr"#).
  • #"H"_2"SO"_4# is a weak acid relative to #"HNO"_3#. In water, the pKa of #"H"_2"SO"_4# is about #-3# vs. #-1.3# (#"HNO"_3#).
  • #"HF"# is a strong acid relative to acetone. In water, the pKa of #"HF"# is about #3.45# relative to #20# (acetone).

Compare pKa differences first, and then make your judgment.

But Wikipedia does say that an acid with a pKa below #-2# is generally a strong acid. Just remember that it's relative to water.