0.80 g of sample of impure potassium dichromate was dissolved in water and made up to 500 mL solution 25 mL of this solution treated with excess of KI in acidic medium and I2 liberated required 24 mL of a sodium thiosulphate solution. 30 mL of this sodium thiosulphate solution required 15 mL of N/20 solution of pure potassium dichromate. What was the percentage of K2Cr2O7 in given sample ?

0.80 g of sample of impure potassium dichromate was dissolved in water and made up to 500 mL solution 25 mL of this solution treated with excess of KI in acidic medium and I2 liberated required 24 mL of a sodium thiosulphate solution. 30 mL of this sodium thiosulphate solution required 15 mL of N/20 solution of pure potassium dichromate. What was the percentage of K2Cr2O7 in given sample ?

  1. A

    73.5 %

  2. B

    75.3 %

  3. C

    36.75 %

  4. D

    none of these

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    Solution:

    K2Cr2O7 + KI   I2 + Cr3+

    I2 + Na2S2O3    I2 + S4O62-

    Na2S2O3 + K2Cr2O7   

    Meq of Na2S2O3      Meq. of  K2Cr2O7 

    30 × N = 15 × 120 N = 140 Meq. of I2 = Meq. of Hypo Meq. of I2 = Meq. of KI Meq of KI = Meq. of K2Cr2O7 24 × 140 = Meq. of 25 mL K2Cr2O7 Meq. of 500 mL K2Cr2O7 = 2440 × 50025 w × 6294 × 1000 = 12             w = 0.588 % purity = 0.5880.8 × 100 = 73.5%

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