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 $NaOH$

Addition of $Na_2CO_3$ to a solution of an oxide in water produces $CO_2$. This experiment indicates that

The $pK_a$ value of the strongest acid among the following is:

Identify the conjugate acid-base pair respectively from the following equilibrium reaction.
$HPO_{4(aq)}^{2-} + H_2O_{(\ell)} \rightleftharpoons PO_{4(aq)}^{3-} + H_3O_{(aq)}^{+}$

The enthalpies of neutralization of $HCN$ and $H_2S$ with a strong base are $12.13 \ kJ \ equi^{-1}$ and $15.9 \ kJ \ equi^{-1}$ respectively. Then:

Three reactions involving $H_2PO_4^-$ are given below :
$(i) \; H_3PO_4 + H_2O \rightarrow H_3O^{+} + H_2PO_4^-$
$(ii) \; H_2PO_4^- + H_2O \rightarrow HPO_4^{2-} + H_3O^{+}$
$(iii) \; H_2PO_4^- + OH^{-} \rightarrow H_2O + HPO_4^{2-}$
In which of the above does $H_2PO_4^-$ act as an acid?