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ISC Board Exam 2014 : Chemistry Paper 2 (Practical) with Model Answers

9 pages, 17 questions, 2 questions with responses, 3 total responses,

ISC 12th
Indian School Certificate Examination (ISC), New Delhi

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CHEMISTRY PAPER 2 (PRACTICAL) Attempt all questions. Question 1 [8] You are provided with two solutions as follows: C-10 is a solution prepared by dissolving 25 gms of sodium thiosulphate crystals (Na2S2O3 xH2O) per litre. C-11 is a solution prepared by dissolving 5 gms of potassium dichromate (K2Cr2O7) per litre. [ PROCEDURE: Rinse and fill the burette with the given solution C-10 (Na2S2O3.xH2O). Pipette out 20 ml or 25 ml of solution C-11 (K2Cr2O7) in a clean conical flask. To this, add 20 ml of C-12 (dilute sulphuric acid) and about 20 ml of C-13 (10% solution of potassium iodide). Now add about 20 ml of distilled water, followed by a pinch of sodium hydrogen carbonate (C-14). Shake the contents of the flask and cover it with a watch glass. Allow the solution to stand for about 5 minutes till the solution becomes dark reddish brown. Titrate the solution by running C-10 from the burette till the solution turns yellowish green. Add about 2 to 3 ml freshly prepared starch (C-15). The colour of the solution changes to dark blue. Continue adding C-10 drop-wise till addition of one drop of C-10 changes the colour of the solution from blue to light green. Repeat the above procedure of titration to get at least two concordant readings. Tabulate your readings. State: (a) The capacity of the pipette used. (b) The titre value you intend to use in your calculations. Show the titre value to the Visiting Examiner. The equations for the above reactions are as follows: K2Cr2O7 + 7H2SO4 + 6KI 4K2SO4 + Cr2(SO4)3 + 7H2O + 3I2 6Na2S2O3 + 3I2 3Na2S4O6 + 6NaI. 2 Cr2O 7 + 14H+ + 6 I 2Cr3+ + 7H2O +3I2 2 2 6S2O 3 + 3I2 3S4O 6 + 6 I Relative atomic masses: K =39 Cr = 52 Na = 23 S = 32 O = 16 56 H=1

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Additional Info : ISC 2014 : Board Model Answers (Chemistry Paper 2)
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