Which of the following is an acidic oxide?

When $NH_3$ reacts with $Cl_2$,which of the following statements are correct?
$(a)$ $NCl_3$ is formed when $Cl_2$ is in excess.
$(b)$ $N_2$ is formed when $Cl_2$ is in excess.
$(c)$ $N_2$ is formed when $NH_3$ is in excess.
$(d)$ $NCl_3$ is formed when $NH_3$ is in excess.

Extra pure $N_2$ can be obtained by heating

Heating $(NH_4)_2Cr_2O_7$ liberates a gas. The same gas is obtained by:

Which is the most thermodynamically stable allotropic form of phosphorus?

Phosphorus reacts with chlorine gas to give a colourless liquid,which fumes in moist air to produce $HCl$ and