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
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.