WBJEE 2024 Physics & Chemistry Paper

The equivalent weight of a substance is the weight of the substance that reacts with or is equivalent to one mole of hydrogen ions or one mole of electrons. In this reaction, the oxidation state of sulfur in Na2S2O3 is +2 , while in Na2S4O6 it is +2.5 . This means that each sulfur atom in Na2S2O3 loses 0.5 electrons. Since there are two sulfur atoms per molecule of Na2S2O3, the total number of electrons lost per molecule is 1 . Therefore, the equivalent weight of Na2S2O3 is equal to its molecular weight divided by the number of electrons lost per molecule, which is M∕1 or simply M.
So the answer is Option B.