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I've studied pka of H2O as 15 and anything with a pka lower than it should be a acid . Guanidine has a pka of 13.8 and is being called as strong base . Why is it so. https://en.m.wikipedia.org/wiki/Guanidine We know that ammonia is a weak base than why is its pka much higher than Guanidine https://en.m.wikipedia.org/wiki/Ammonia

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  • $\begingroup$ Equivalent resonance in amidine bases $\endgroup$ – YUSUF HASAN Feb 16 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 at 17:13
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Wikipedia notes that if a base (in water solution) has a conjugate acid whose $pK_a$ 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 at 8:13
  • $\begingroup$ Conjugate acid pKa < 13. $\endgroup$ – Oscar Lanzi Feb 19 at 9:59
  • $\begingroup$ But Wikipedia states pka of Guanidine<pka of ammonia $\endgroup$ – Harsh jain Feb 19 at 15:08
  • $\begingroup$ 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. $\endgroup$ – Oscar Lanzi Feb 19 at 18:36

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