# Why Guanidine is called as strong base

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

• Equivalent resonance in amidine bases – YUSUF HASAN Feb 16 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 at 17:13

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.