A salt X on strong heating gives brown fumes, and a black residue which dissolves in dilute $H X_{2} SO X_{4}$ to give a blue solution. The original salt gives a blue-green flame. Considering the complete decomposition of one mole of X, the identity of X and the total number of moles of gaseous products formed are respectively

Answer: C

Brown fumes show a nitrate, black residue soluble to give a blue solution is CuO, and the blue-green flame is

Cu X^{2 +}

. The equation

2 Cu (NO X_{3}) X_{2} 2 CuO + 4 NO X_{2} + O X_{2}

gives 5 mol gas for 2 mol salt, i.e. 5/2 = 2.5 mol per mole.

Pb (NO X_{3}) X_{2}

has the same gas count but gives a yellow residue and no blue-green flame.