Question

What is the pKa for ammonia, given that the pKb for ammonia is 4.7

Answer 1

Ammonia pKb=4.7 Since pKa + pKb=14, the pKa will be 9.3

Answer 2

$pKa+pKb=pKw\\ pKw=14\\ pKa=pKw-pKb\\ pKa=14-4,7\\ pKa=9,3$