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
