# Why is the degree of dissociation for a weak acid α, but for a weak base, it's 1−α?

From the equation used in this answer: Calculating charge on amino acid from pKa

$$\alpha=\frac{1}{10^{\mathrm pK_\mathrm a-\mathrm{pH}}+1}$$

Degree of dissociation of weak acid is $\alpha$. However, when it comes to weak base, degree of dissociation is $1-\alpha$. What is the explanation behind this?

• I think this is because $\alpha$ is defined from $\ce{HA <=> A- + H+}$; which for a base would be exactly reversed. Not sure enought about it to answer, though. – Jan Oct 2 '16 at 22:28
• @Jan is almost certainly correct. For consistency, we looking at the equation he wrote. Same reason we generally look at pH instead of pOH. – Zhe Nov 20 '16 at 20:45