Indian National Chemistry Olympiad Examination 1999
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* This question paper has 9 pages.
An amount of commercial vinegar containing 75mg of acetic acid (PKa = 4.75) is diluted to 10 cm3 . The resulting solution is titrated against 0.1 M NaOH using a suitable indicator.
- Calculate the pH of the solution before the addition of alkali.
- At what point in the titration is the pH of the reaction mixture equal to the pKa value of the acid?
- Calculate the pH of the reaction mixture at the equivalence point.
- What is the pH of the reaction mixture when 0.1 cm3 of 0.1M of NaOH is added beyond the equivalence point?
- Which indicator is most suitable for these titration? What is the pH range in which the indicator can be used?
- Which indicator is most commonly used for the titration? Why does the colour change occur at the equivalence point?
- If 0.1M NaOH titrant is replaced by 0.1M NH4OH, what will be the pH of the reaction mixture at the equivalence point? (pKb of NH4OH = 4.75). Explain why a visual indicator cannot be used in this titration. Suggest a reliable instrumental technique which can be used for such titrations.
The Phosphorous content in a fertilizer can be determined absorptimetrically.
Phosphorus (atomic weight = 31) in a mixed fertilizer sample was suitably treated and the resulting solution absorbed 53.80% radiation of wavelength 830 nm. The radiation travelled a distance of 1 cm through the solution.
A 0.6 ppm Phosphorus containing solution showed a transmittance of 35.12% at the same wavelength.
- What is the concentration of the Phosphorus in the fertilizer solution in moles dm-3 ?
- Calculate the molar absorbtivity of the fertilizer solution
The hydrolysis of ATP (Adenosine triphosphate) to ADP (Adenosine diphosphate) and P (Phosphate) is an important biochemical reaction, with standard free energy change given below: ATP + H2O -----> ADP + P DG0298.15K = -29.3 KJ/mol
The symbols ATP, ADP & P in the above equation stand for different ionised & unionised forms of these substances. However, we will ignore such complications. The standard state for ATP is defined as the state where the sum of concentrations of all forms of ATP is equal to 1 mol/L and with all activity coefficients equal to unity. The same is true for ADP and P.
- Find the equilibrium constant for the above reaction.
- If the initial concentration of ATP is 0.01 mol/L, what will be the equilibrium concentrations of the products?
- The ADP formed in the above reaction can be recycled. One way of regenerating ATP is by coupling the reaction with the hydrolysis of PEP (Phosphoenolpyruvic acid) to Py (pyruvate ion) and P (Phosphate). The reactions are represented as follows:
ADP + P <-----> ATP + H2O D G0298.15K = + 29.3 KJ/mol
Given initial concentrations of PEP and ADP equal to 0.01 mol/L, calculate the concentrations of ATP produced at 298.13 K.
- Another way to regenerate ATP is through enzymatic reaction. The molecular details of this reaction are complicated, but we assume that this is equivalent to one step reaction, that is,
PEP + ADP <-----> ATP + Py
Calculate the amounts of ATP regenerated starting with the same initial concentrations of PEP and ADP stated in part C. Compare the amounts of ATP obtained in parts C & D. State which of the two reactions is more efficient in regenerating ATP.
A research student of chemistry is studying the decomposition reaction of N2 O5
2N2O5 <-----> 4NO2 + O2
The following data are obtained by her:
- Plot a graph of log P(N2 O5) Vs t. What does the graph suggest about the order of the reaction?
- Estimate the value of the rate constant from the graph and also calculate t½
- The following mechanism is suggested for the above reaction
Derive the expression for rate law which is consistent with the observed kinetic data? (Assume that the concentrations of the intermediate species are negligible during the reaction.)
When a neutron is absorbed by U238, the resulting U239 undergoes two successive beta decays to yield Pu239. The reactions are
92U239 ® -1 b 0 + 93 Np239
93 Np239 ® -1 b 0 + 94 Pu 239
The half life of the first decay is 23 mins and that of second decay is 56 hrs. This reaction is of great importance in the context of breeding the fissile Plutonium from natural Uranium. A sample of natural Uranium is irradiated by neutrons for 15 mins. Make suitable approximations and estimate the ratio of Neptunium to Plutonium atoms after a week.
A sample of fuel oil has C/H ratio 9.33 (by weight) and contains 1.37% sulphur by weight. The net calorific value of the fuel oil is 9478.74 kcal/kg at 25oC. What is the gross calorific value? Latent heat of water is 583.2 kcal/kg.
An effluent sample from a formaldehyde plant is known to contain methanol and formaldehyde. The analysis of the solution indicated that the total organic carbon content (TOC) and theoretical oxygen demand (ThOD) are 258.3 mg/L and 956.3 mg/L respectively. Find the concentration of each of the compounds in the sample.
In 1874, Wilhelm Koerner pointed out that if all the positions on a benzene ring are equivalent, there should exist only three disubstituted benzene namely, ortho, meta and para. These should be distinguished by the numbers of isomers each would yield when an additional substituent is introduced.
- You are given three bottles. Each contains one of the three xylenes. Show how Koerner's method could be used to distinguish between the three by the process of mononitration.
- Could Koerner's method be used to distinguish between the three possible trimethyl benzenes? Explain.
- Can the same be applied to distinguish o-bromochlorobenzene and m-bromochlorobenzene? Explain.
A certain organic compound A of unknown origin was analysed as follows:-
I CHEMICAL ANALYSIS DATA
- A is liquid and gives a positive Beilstein test.
- With hot alcoholic KOH, A gives B.
- B gives a negative Beilstein test.
- Ozonolysis of B followed by mild oxidation gives two acids of equivalent weights 74 and 46 respectively.
- A mild permanganate oxidation of B gives a cis diol.
II RELATIVE MOLECULAR MASS DATA
1.85 g of A is produced on evaporating 535.3 cm3 of vapour at 342K and 730mm Hg (97.3 KPa).(Gram molecular volume is 22.4dm3 at STP.)
III PHYSICAL DATA
- A is optically active
- Schematic mass spectrum of A
i) From chemical analysis data, deduce possible structures for A and B
ii) From relative molecular mass data, deduce the molecular mass of A.
iii) From physical data, identify A.
Explain your reasoning in all the cases.
Answer the following, selecting one from the multiple options given.
- One can make__________ different polypeptides of 10 residues from the 20 naturally occuring amino acids. a) 1020 b) 2010 c) 10*20
- For 50 ml of 10 mM fully protonated histidine,_____ of a base must be added to bring the histidine solution to a pH equivalent to its pI. a) 1 micromole b) 1 millimole c) 20 mmoles
- The amide hydrogen of every peptide bond in ______ protein is hydrogen bonded. a) An a-helical b) a globular c) a b -sheet containing
- Protein biosynthesis occurs ____________. a) from N-terminal to C-terminal b) from C-terminal to N-terminal c) from both the terminals.
- The protecting group, ______________ is used for carboxy-terminal protection. a) tert-butyloxycarbonyl b) tert-butyl c) trifluoroacetyl
- Briefly explain why most of granular proteins in solution do the following:
a) Precipitate at low pH.
b)Increase in solubility, then decrease in solubility and finally precipitation as the ionic strength is increased from zero to high value.
c) Precipitate on heating.
d) Decrease in solubility as the dielectric constant of the medium is decreased by addition of a water-miscible non-polar solvent.
e) Denaturation if the dielectric constant is decreased substantially (non-polar solvent).
- A mixture of alanine, glutamic acid and arginine was chromatographed on a weakly basic ion-exchange column at pH 6.1
a) Write solution structures of these amino acids at pH 6.1.
b) Predict the order of elution of the amino acids from the ion-exchange column.
c) Are the amino acids separated from each other? Explain.
d) Suppose you have a weakly acidic ion-exchange column, also at pH 6.1. Predict the order of elution of the amino acids from this column.
e) Propose a strategy to separate the amino acids using one or both columns. Explain your rationale (Assume only ionic interactions).
- Transition metal ions show strong tendency to form coordination complexes. Amongst them, the largest number of complexes are formed by cobalt (III). This is because cobalt (III) octahedral complexes are kinetically inert, hence it is possible to substitute ligands in an existing complex. Most of the complexes formed by cobalt (III) complex are octahedral with coordination number = 6, rather than tetrahedral with coordination number = 4. The most commonly used method for the preparation of octahedral cobalt (III) complexes involve oxidation of a solution containing Co2+ with air or H2O2 in the presence of appropriate ligands and a catalyst such as activated charcoal. For example:
CoCl2 aq + NH3 aq + NH4Cl [O] ® [Co(NH3)5Cl] Cl2.
In [Co(NH3)5Cl] Cl2 complex one of the coordinated ammonia molecule can be substituted with Cl- by the reaction of HCl & NaCl and can be precipitated as perchlorate salt by addition of sodium perchlorate. The resulting complex A when dissolved in water and gently heated isomerised to species B. The complex A on reaction with Cl- gave only one isomer of complex product Co(NH3)3Cl3, while B on reaction with Cl- yielded a pair of isomers of Co(NH3)3Cl3. Based on the above explain the following:
a) Why aqueous solution of cobalt (II) salts are used as a starting material rather than aqueous solution of cobalt (III) salts for the preparation of cobalt (III) complexes ?
b) Why cobalt (II) undergoes ready oxidation to corresponding cobalt (III) complex ?
c) Why cobalt (III) shows preference to form octahedral rather than tetrahedral complexes?
d) Draw structures of A and B.
e) Draw and name the isomer of the products, Co(NH3)3Cl3 , produced from A.
f) Draw and name the isomers of the products, Co(NH3)3Cl3, formed from B.
g) How can one distinguish isomers of Co(NH3)3Cl3, with the help of 1H nmr spectroscopy ?
- The formula Co(NH3)4 CO3 Cl could be that of three isomers. Write their possible structures. How can one distinguish the coordination isomers using (i) chemical test and (ii) instrumental techniques (such as ir, 1H nmr and conductivity).