ISC Class XII Exam Question Papers 2012: Chemistry Practical

CHEMISTRY Paper – 2

(PRACTICAL)

(Three hours)
(Candidates are allowed additional 15 minutes for only reading the paper. They must NOT start writing during this time.)

ALL ANSWERS MUST BE WRITTEN IN HE ANSWER BOOKLET PROVIDED SEPARATELY

Question 1 is an oxidation-reduction titration in which sufficient working details are given.
All essential working must be shown.

Question 2 is an exercise dealing with identification of organic compounds Credit will be given for precise observations recorded and for well-drawn deductions.
Question 3 is an exercise in qualitative analysis.

Mathematical Tables are provided.

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Read the questions carefully and follow the given instructions.
Attempt all questions.
All working, including rough work, should be done on the same sheet as the rest of the answer. The intended marks for questions or parts of questions are given in brackets I.

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You are provided with the solutions are follows:

(a)  C-10 is a solution prepared by dissolving 6 gms of an impure sample of potassium dichromate (K₂Cr₂0₇) per litre of the solution.

(b)C-11 is a solution of sodium thiosulphate (Na₂S₂0₃.5H₂0) containing 25 gms per litre of the solution.

PROCEDURE:

Rinse and fill the burette with the given solution C-11 (Na₂8₃0₃.5H₂0). Pipette out 20 ml or 25 ml or the solution C-10 (K₂Cr₂0₇), into a clean conical flask. To this, add 20 ml of dilute sulphuric acid C-12 and about 20 ml of potassium iodide solution (10%) C-13. Now, add about 20 ml of distilled water followed by a pinch of sodium hydrogen carbonate NaHCO₃ C-14. Shake the contents of the flask and cover it with a piece of paper. Allow the solution to stand for about 5 minutes till the solution becomes dark reddish brown.

Titrate this solution by running C-11 from the burette till the solution turns yellowish green. Add about 2 to 3 ml of freshly prepared starch C-15. The colour of the solution changes to dark blue. Continue adding C-11 dropwise till the addition of one drop of C­11 changes the colour from blue to light green.

Repeat the above procedure of titration to get concordant readings. Tabulate your readings.

State:

(i)     The capacity of the pipette used.

(ii)   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: K₂Cr₂0₇ + 7H₂SO₄ + 6KI —> 4K₂SO₄ + Cr₂(SO₄)₃+ 7H₂0 + 312 6Na₂S₂0₃+ 31₂—> 3Na₂S₄0₆ + 6 Na!

OR

Cr₂0₇²– + 14H⁺ + 61—> 2Cr³⁺ +7H₂0 + 312

6S₂0₂²– + 31₂—> 3S₄0₆²– + 61

Relative atomic masses:

K= 39 Cr = 52 Na = 23 S= 32 0=16 and H = 1

Calculate the following:

(i)The molarity of sodium thiosulphate (Na₂S₂0₃.5H₂0) solution C-11.

(ii)The molarity of potassium dichromate (K₂Cr₂0₇) solution C-10.

(iii)The strength of potassium dichromate solution in gms per litre.

(iv)The percentage purity of the sample of potassium dichromate.

Question 2

You are provided with two organic compounds, C-16 and C-17.

Perform the experiments given below on each of the two compounds. Record the changes taking place at every step of the experiment.

Note the smell of the substance formed, if significant, the colour of the solution obtained, the colour of the precipitate produced an any other observations you may have. State the identity of each compound on the basis of the experiments and observational changes.

(a) Substance C-16

1.  Take 2 ml of C-16 in a test tube and add 1 ml of sodium hydroxide solution. Boil the contents.
2.  Take 2 ml of C-16 in a test tube and add 2 ml of lead acetate solution. Shake. Add 1 ml of ammonium hydroxide solution.
3.  Take 2 ml of C-16 in a test tube and add 2 ml of Fehling’s solution. Warm the contents.
4.  Take 2 ml of C-16 in a test tube. To this, add a pinch of resorcinol and 1 ml. of concentrated hydrochloric acid. Shake the contents and warm the test tube.

Show the results as required to the Visiting Examiner.

(b) Substance C-17

1. Take 1 ml of C-17 and add 5 ml of bromine water. Shake.
2.  Take 1 ml of C-17 in a dry test tube, add a pinch of solid sodium nitrite to it followed by 3 to 4 drops of concentrated H₂SO₄. Shake the contents of the test tube. Note the colour. Add 3 to 4 drops of water to this followed by aqueous sodium hydroxide solution till it becomes alkaline.
3.  Take 1 ml of C-17 in a test tube and add 1 ml of neutral FeCI₃ solution. Show the results as required to the Visiting Examiner.

Question 3

Analyse qualitatively the substance C-18 which contains two anions and two cations. Identify these ions.

(a) While testing for anions you must mention:

1. How the solution/soda extract was prepared.
2. How the gases were identified.
3. The confirmatory test for each anion.

Show the results as required to the Visiting Examiner.

(b)While testing for cations you must mention:

1. How the original solution for group analysis was prepared.
2. The formal group analysis with pertinent group reagents.
3. The confirmatory test for each cation.

Show the results as required to the Visiting Examiner.

(1)  Use of qualitative analysis booklets/tables is not allowed.

(2)  Dry tests are not accepted as confirmatory tests.

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