# What is the pka range for a strong acid?

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!

• $\text{HI}$ is a strong acid relative to water. In water, its pKa is about $- 10$ vs. $15.7$ (water). ${K}_{a} \approx 3.2 \times {10}^{9}$
• ${\text{H"_2"SO}}_{4}$ is a strong acid relative to water. In water, its pKa is about $- 3$ vs. $15.7$ (water). ${K}_{a} \approx {10}^{3}$
• $\text{HF}$ is a weak acid relative to water. In water, its pKa is about $3.17$ relative to $15.7$ (water). ${K}_{a} = 7.2 \times {10}^{- 4}$

Yet...

• $\text{HI}$ is a weak acid relative to $\text{HBr}$. In water, the pKa of $\text{HI}$ is about $- 10$ vs. $- 9$ ($\text{HBr}$).
• ${\text{H"_2"SO}}_{4}$ is a weak acid relative to ${\text{HNO}}_{3}$. In water, the pKa of ${\text{H"_2"SO}}_{4}$ is about $- 3$ vs. $- 1.3$ (${\text{HNO}}_{3}$).
• $\text{HF}$ is a strong acid relative to acetone. In water, the pKa of $\text{HF}$ 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.