# How do you calculate a Ka value from pKa?

$p {K}_{a}$ $=$ $- {\log}_{10} {K}_{a}$. Thus ${K}_{a}$ $=$ ${10}^{- p {K}_{a}}$
Remember your definitions for logarithmic functions. When we write ${\log}_{a} b = c$, we specify that ${a}^{c} = b$. For example ${\log}_{10} 100 = 2$, and ${\log}_{10} 1000 = 3$. It follows that exceptionally strong acids have NEGATIVE $p {K}_{a} \text{'s}$.