Arsenic Oxide (As₂O₃) Is Used as a Standard in Oxidation-Reduction

Arsenic oxide (As₂O₃) is used as a standard in oxidation-reduction titrations because it is stable and reacts very predictably. Reaction with MnO₄^- in acidic solution gives aqueous arsenic (H₃AsO₄) and Mn²⁺. If the coefficient of MnO₄^-is 4 in the balanced chemical equation of reaction, the coefficient of H₃AsO₄ is

A) 2.
B) 4.
C) 5.
D) 8.
E) 10.