Which one is not an acid salt?

Which of the following compounds are diprotic?

Four species are listed below:
$i. \, HCO_3^-$
$ii. \, H_3O^{+}$
$iii. \, HSO_4^-$
$iv. \, HSO_3F$
Which one of the following is the correct sequence of their acid strength?

$NH_4OH$ is a weak base because

The nitrating mixture contains concentrated $H_2SO_4$ and concentrated $HNO_3$. In this mixture,$HNO_3$ acts as a:

Which one of the following compounds is not a protonic acid?