Ammonium dichromate is heated and the swollen green residue is then fused in a borax bead. The number of electrons transferred per formula unit of the salt in its decomposition (oxidation states of N and Cr taken as -3 and +6 initially) and the colour of the bead are

Answer: D

(NH X_{4}) X_{2} Cr X_{2} O X_{7} Cr X_{2} O X_{3} + N X_{2} + 4 H X_{2} O

. Two Cr go from +6 to +3, gaining 6 electrons, while two N go from -3 to 0, losing 6 electrons, so 6 electrons are transferred. The residue is

Cr X_{2} O X_{3}

, which forms green chromium(III) metaborate in both flames.