Which of the following statements about iodometry and iodimetry are correct?

A1 mol of $I X_{2}$ reacts with 2 mol of $S X_{2} O X_{3} X^{2 -}$ to form $S X_{4} O X_{6} X^{2 -}$
B1 mol of $Cr X_{2} O X_{7} X^{2 -}$ liberates 3 mol of $I X_{2}$ from excess acidified KI
CIn iodometry starch should be added at the very start of the titration
DThe n-factor of $Na X_{2} S X_{2} O X_{3}$ in forming $S X_{4} O X_{6} X^{2 -}$ is 1

Answer: A, B, D

Two thiosulphate ions lose two electrons in all, so n = 1 each and 1 mol

I X_{2}

needs 2 mol. Dichromate takes 6 electrons, giving 3

I X_{2}

. Starch is added near the end point (pale yellow), not at the start, because it adsorbs iodine strongly. So A, B and D are correct.