Which of the following molecules has the weakest $(p \pi - d \pi)$ back bonding?

Which of the following contains both covalent and ionic bonds?

Which of the following does not have the correct order of the given property?

The formal charges of $N_{(1)}$,$N_{(2)}$ and $O$ atoms in the structure shown below are respectively:
$:N_{(1)}=N_{(2)}=\ddot{O}:$

Identify the thermal decomposition reactions that evolve a gas containing an equal number of $\sigma$ and $\pi$ bonds.
$(a)$ $BeCO_3 \xrightarrow{\Delta } BeO + CO_2$
$(b)$ $Fe_2(SO_4)_3 \xrightarrow{\Delta } Fe_2O_3 + 3SO_3$
$(c)$ $FeSO_4 \xrightarrow{\Delta } Fe_2O_3 + SO_2 + SO_3$
$(d)$ $(NH_4)_2Cr_2O_7 \xrightarrow{\Delta } Cr_2O_3 + N_2 + 4H_2O$
$(e)$ $ZnSO_4 \xrightarrow{\Delta } ZnO + SO_3$
$(f)$ $NH_4Cl \xrightarrow{\Delta } NH_3 + HCl$

Observe the following sets:

Label	Order	Property
$i.$	$NH_3 > H_2O > SO_2$	Bond angle
$ii.$	$H_2O > NH_3 > H_2S$	Dipole moment
$iii.$	$N_2 > O_2 > H_2$	Bond enthalpy
$iv.$	$NO^{+} > O_2 > O_2^{2-}$	Bond order

Which of the above sets are correctly matched?