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I've studied $\mathrm{p}K_\mathrm{a}(\ce{H2O}) = 15,$ and anything with a $\mathrm{p}K_\mathrm{a}$ lower than should be an acid. Guanidine has a $\mathrm{p}K_\mathrm{a}$ of $13.8$ and is being called by Wikipedia a strong base. Why is it so?

According to Wikipedia, ammonia is a weak base. Then why is its $\mathrm{p}K_\mathrm{a}$ much higher than the $\mathrm{p}K_\mathrm{a}$ of guanidine?

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  • $\begingroup$ Equivalent resonance in amidine bases $\endgroup$ Feb 16 '19 at 17:08
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    $\begingroup$ It's pKa of conjugated acid ($\ce{pK_{bH^+}}$) - it's protonated form is about as acidic as water. $\endgroup$
    – Mithoron
    Feb 16 '19 at 17:13
  • $\begingroup$ due to the stability of carbocation formed after protanation $\endgroup$
    – user102709
    Dec 27 '20 at 12:44
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Wikipedia notes that if a base (in water solution) has a conjugate acid whose $\mathrm{p}K_\mathrm{a}$ exceeds $13,$ it's a strong base. The criterion is (apparently) based on complete or at least predominant dissociation up to $0.1$ molar. Guanidine makes it, ergo a strong base.

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    $\begingroup$ We know that ammonia is a weak base than why is its pka much higher than Guanidine en.m.wikipedia.org/wiki/Ammonia $\endgroup$
    – Harsh jain
    Feb 19 '19 at 8:13
  • $\begingroup$ Conjugate acid pKa < 13. $\endgroup$ Feb 19 '19 at 9:59
  • $\begingroup$ But Wikipedia states pka of Guanidine<pka of ammonia $\endgroup$
    – Harsh jain
    Feb 19 '19 at 15:08
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    $\begingroup$ No. The number for guanidine is for its conjugate acid while the number for ammonia is for NH3 itself, the conjugate acid for ammonia would be about pKa=5. Conjugate acid pKa is what counts for identifying a strong base. It's inconsistent nomenclature, yes that sucks. $\endgroup$ Feb 19 '19 at 18:36

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