# Why is guanidine considered a strong base?

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?

• Equivalent resonance in amidine bases – Yusuf Hasan Feb 16 '19 at 17:08
• It's pKa of conjugated acid ($\ce{pK_{bH^+}}$) - it's protonated form is about as acidic as water. – Mithoron Feb 16 '19 at 17:13
• due to the stability of carbocation formed after protanation – user102709 Dec 27 '20 at 12:44

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.