Which one of the following combines with $Fe(II)$ ions to form a brown complex?

The correct sequence of decrease in the bond angle of the following hydrides is

Which of the following compounds does not exist?

All alums contain ......

Phosphorus is manufactured by heating in an electric furnace a mixture of

Account for the following observations:
$(A)$ $AlCl_3$ is a Lewis acid.
$(B)$ Though fluorine is more electronegative than chlorine yet $BF_3$ is a weaker Lewis acid than $BCl_3$.
$(C)$ $PbO_2$ is a stronger oxidizing agent than $SnO_2$.
$(D)$ The $+1$ oxidation state of thallium is more stable than its $+3$ state.