The conjugate base of hydrazoic acid is

Which of the following is a Lewis acid?

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

The pair of compounds which cannot exist together in aqueous solution is:
$I.$ $NaH_2PO_4$ and $NaHCO_3$
$II.$ $Na_2CO_3$ and $NaHCO_3$
$III.$ $NaOH$ and $NaH_2PO_2$
$IV.$ $NaHCO_3$ and $NaOH$

Which of the following statements is correct?

Which of the following compounds are diprotic?