Calculate the concentration of hypo (Na2S2O3.5H­2O) solution in g dm–3 if 10.0 of this solution decolourised 15 mL of M/40 iodine solution.


Correct option is

18.6 g dm–3


The balanced equation for the redox reaction is


For the balanced equation, it is evident that

       2 moles of Na2S2O3 ≡ 1 mole of I2

Applying molarity equation, we have





Mol. Mass of Na2S2O3.5H2O = 248 g mol–1

                                = 18.6 g dm–3     



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0.5 g of an impure sample of oxalate was dissolved in water and the solution made to 100 ml. On titration 10 ml of this solution required 15 ml of N/20 KMnO4 solution. Calculate the percentage of pure oxalate in the sample.


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