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Question

In iodometric titrations, an oxidizing agent such as KMnO4,K2Cr2O7,CuSO4,H2O2 is allowed to react in neutral medium or in acidic medium with excess of potassium iodide to liberate free iodine (I2).
Free iodine is titrated against standard reducing agent usually with sodium thiosulphate. The reactions are as follows:
K2Cr2O7+6KI+7H2SO4Cr2(SO4)3+4K2SO4+7H2O+I2
2CuCO4+4KICu2I2+2K2SO4+I2
I2+Na2S2O32NaI+Na2S4O6
In iodometric titrations, starch solution is used as an indicator. Starch solution gives blue or violet colour with free iodine. At the end point, blue or voilet colour disappears when iodine is completely changed to iodide.

What volume of 0.40 M Na2S2O3 would be required (in mL) to react with I2 liberated by adding 0.04 mole of KI to 50 mL of 0.20 M CuSO4 solution?

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Solution

meq. of KI meq. of CuSO4 meq. of I2 meq meq. of Na2S2O3
mequivalent of KI=0.04×103=40
mequivalent of CuSO4=50×0.2=10

limiting reagent is CuSO4
meq. of Na2S2O3=10M×x×VmL=10
0.4×VmL=10VmL=25 mL

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