In the standardisation of N2S2O3 using K2Cr2O7 by iodometry, the equivalent weight of K2Cr2O7 is

Question

A) Molecular weight/2

B) Molecular weight/6

C) Molecular weight/3

D) Same as molecular weight

Answer 1

Answer: The following reaction occur between S2O3^(2-) and Cr2O7^(2-) : 
26H+ + 3S2 O3^(2-) + 4Cr2O7^(2-) → 6SO3^(2-) + 8Cr3+ + 13H2O 
Change in oxidation number of Cr2O4^(2-) per formula unit is 6 (it is always fixed for Cr2O7^(2-)). 
Hence, equivalent weight of K2Cr2O7 = (Molecular weight)/6

Answer 2

Answer is (molecular weight)/6

Potassium Dichromate is used to standardize Sodium Thiosulfate solutions. In acidic solution, Potassium Dichromate oxidizes Potassium Iodide to Iodine; the Iodine thus generated can then be titrated with the Thiosulfate solution using Starch Indicator: 

Cr2O72- + 6 I- + 14 H+ → 2 Cr3+ + 3 I2 + 7 H2O 

Since each molecule of it is releasing 6 electrons its equivalent wt is m.wt/6 
 

K2Cr2O7 Cr is in 6+ it is converted in to Cr3+ that is accepting 3 e- for two Cr3+ 6 e-s.