In the reaction, P + Q $$\to$$ R + S, the time taken for 75% reaction of P is twice the time taken for 50% reaction of P. The concentration of Q varies with reaction time as shown in the figure. The overall order of the reaction is

Concentrated nitric acid, upon long standing, turns yellow-brown due to the formation of

The arrangement of X^$$-$$ ions around A⁺ ion in solid AX is given in the figure (not drawn to scale). If the radius of X^$$-$$ is 250 pm, the radius of A⁺ is

Upon treatment with ammoniacal H₂S, the metal ion that precipitates as a sulphide is