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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?

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    $\begingroup$ 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. $\endgroup$
    – Jan
    Commented Oct 2, 2016 at 22:28
  • $\begingroup$ @Jan is almost certainly correct. For consistency, we looking at the equation he wrote. Same reason we generally look at pH instead of pOH. $\endgroup$
    – Zhe
    Commented Nov 20, 2016 at 20:45

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