Which of the following is the structural formula of hypophosphorous acid?

In some of the reactions,thallium resembles aluminium,whereas in others,it resembles group-$I$ metals. Support this statement by giving some evidence.

Which of the following nitrogen trihalides is the weakest base?

Give the correct characteristic colours of the products $(P, Q, R, S)$ formed in the following reactions:
$(A)\ NO + NO_2 \xrightarrow{< 250 K} P$
$(B)\ 2 NO_2 \longleftrightarrow Q$
$(C)\ Pb(NO_3)_2 \xrightarrow{673 K} R + PbO$
$(D)\ 4 HNO_3 + P_4O_{10} \longrightarrow S + 4 HPO_3$

What is the molecular formula of pyrophosphoric acid?

Which compound is related to Haber's process?