Question

why is aqueous solution of ammonia weakly basic?

Answer 1

Ammonia (NH3) have a loan  pair in N group 
So, it accept a proton from water (H2O )
then it forms Ammonium ion and hydroxide ion

NH3 + H2O ----> NH4+    +   OH- 

But the concentration of OH - ion is less so it is a weal base