Question

Sodium thiosulphate (Na2S2O3 .5H2O) reacts with
iodine as follows:
2S2O3 + I2
S4O6 +2I
Calculate the equivalent mass of Na2S2O3.
​

Answer 1

Answer:

Equivalent mass of  Na₂S₂O₃ = 158 g

Explanation:

The reaction is,

2Na₂S₂O₃ + I₂ → Na₂S₄O₆ + 2NaI 

• Oxidation of S in S₂O₃⁻² = 2 
• Oxidation number of S in S₄O₆⁻² = 5/2

Again,    

2S₂O₃⁻² → S₄O₆⁻² 

For 2 moles of S₂O₃⁻²,

Change in oxidation number = (4x5)/2 - (2x2x2) = 10-8 = 2

                                                 

For 1 mole of  S₂O₃⁻²,

Change in oxidation number = 2/2 = 1 

∴,

Equivalent mass of  Na₂S₂O₃

= molar mass / change in oxidation number

= 158/1

= 158

Equivalent mass of  Na₂S₂O₃ = 158 g