Which of the following salts is acidic?

$(i)$ $A$ strong acid has a weak conjugate base.
$(ii)$ An acid is an electron pair acceptor.
The above statements $(i)$ and $(ii)$ are:

Ammonium hydroxide $(NH_4OH)$ is a:

Which one of the following statements is correct?

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?

The total number of diprotic acids among the following is:
$H_3PO_4, H_2SO_4, H_3PO_3, H_2CO_3, H_2S_2O_7, H_3BO_3, H_3PO_2, H_2CrO_4, H_2SO_3$