(35) An important redox reaction used to determine trace amounts of ozone and other oxidants in polluted air involves oxidation of iodide ion:

The solution is acidified and the I2 is titrated with sodium thiosulphate solution using starch indicator. One litre of a mixture of O3 and O2 at NTP was allowed to react with the excess of KI and I2 liberated required 40ml. of M/10 sodium thiosulphate solution. What is the percentage by volume of ozone in the mixture?

Submission: Closed

Aman's Solution edited by INDIANET

We have 1 lit. mixture of ozone & oxygen and we react it with excess of KI (in aq. medium) which releases iodine gas (along with oxygen and KOH). We then acidify the solution which actually takes all OH- radical (which may interfere later). Please note that oxygen produced is actually the reduced form of oxidising agent (ozone). Liberated iodine is titrated with hypo and this helps us to know the amount of liberated iodine.

I2 + S2O3-- -------> 2I- + S4O6-- (it is not required to balance the equation)

From given data: meq (hypo) = [40 * 1/10]

therefore, meq (iodine gas) = 4 meq (by law of eqv.)

milimoles (iodine gas) = 4/2 = 2mmol

(n-factor for I2-->I- is 2; meq = mmol * n-factor)

therefore from given reaction we get, mmol of ozone = 2 mmol (balanced reaction given)

or, moles of ozone = 2 * 10-3 mol

or, volume of ozone = 22.4 * 2 * 10-3 lit.

= 0.0448 lit.

Total volume of original mixture = 1 lit. Therefore, Volume percentage of ozone = 4.48%

Cracked by:

  1. Aman Verma, 12th Pass, Hans Raj Model School, Punjabi Bagh, New Delhi.
  2. Jithin Prakash, 12'th, Army School, Deolali.

(34) Prove that

is an odd function, where [ ] denotes the greatest integer function.

Submission: Closed

Solution (.pdf)

Cracked by:

  1. Arpit Maheshwari, Class 12th, Campion School, Bhopal (M.P.).
  2. Jithin Prakash, 12, Army School, Deolali.
  3. Premraj Manohar Narkhede, 12, V G Vaze College, Mulund (E), Mumbai.

(33) If y(x-y)2=x , then show that

Submission: Closed

Solution provided by Prashant (page1 page2)

Cracked by:

  1. Umang Merwana, Bhopal (M.P.). Umang has qualified in the screening 2004.
  2. Prashant Patel, 12th pass from Kendriya Vidyalaya, Patna. Prashant had secured rank 593 in NSTSE (National Science Talent Search Examination) year 2002.
  3. Swapneel Subhash Sonawane, 12th, D.G. Ruparel College, Mumbai.
  4. TusharJain, Babu Banarsidas Institute of Technology, Gaziabad, U.P.

(32) Evaluate

Submission: Closed

Answer:

Cracked by:

  1. Yash Agrawal, Class XII, St. Joseph's Co.Ed. School, Bhopal (M.P.). Yash has qualified in the screening 2004 and has scored 90.2% in CBSE 12th.
  2. TusharJain, Babu Banarsidas Institute of Technology, Gaziabad, U.P.
  3. Umang Merwana, Bhopal (M.P.).
  4. Jathin Das, Class XII, Abu Dhabi Indian School, Abu Dhabi, UAE.
  5. Anupam N., B.Tech. 1 Yr., NIT Trichy.
  6. Royan John D'mello, St. Aloysius College, Mangalore.
  7. Swapneel Subhash Sonawane, 12th, D.G. Ruparel College, Mumbai.

(31) A gun is mounted on a gun carriage movable on a smooth horizontal plane. The gun is elevated at an angle a to the horizontal. A shot is fired which leaves the gun in a direction inclined at an angle b to the horizontal. If the mass of gun+carriage is n times that of the shot, show that .

Submission: Closed

Solution (.pdf)

Cracked by:

Rahul Rajanna Bettkote, Class XII, Bishop Cotton Boys' School, Bangalore.

Last modified: Sunday, 27 November 2011, 2:09 PM