pu test1

CHEMISTRY Class

II IIT-JEE Achiever (Integrated) 2011-13 Solution to PU Unit Test - 01

Test date

12-05-2012

PART - A

I. Questions carrying ONE mark each [5 × 1 = 5] 1. State Faraday’s first law of electrolysis. Ans During electrolysis, amount of substance deposited at the electrode is directly proportional to quantity of electricity passed through the electrolyte. 2. What are enantiomers? Ans Enantiomers are the pair of optical isomers that are non-superimposable mirror images of each other. 3. Name the reagent employed to convert bromoethane to ethane. Ans Alcoholic KOH. ether 4. CH 3 CH 2 CH 2 Cl alc KOH  → B HBr  → C Na / → D What is the product ‘D’ in the above reaction? CH3 CH3

Ans 2,3-dimethylbutane or H3C CH CH CH3 CH3CH2CH2 Cl

alc KOH

CH3CH CH2

HBr

H3C CH

CH3

Na/ether

CH3 CH3 H3C CH

CH CH3

Br

5. What is the pH of 10−2 M NaOH? Solution [OH−] = 10−2 pOH = 2 ∴ pH + pOH = 14 pH = 12 PART - B

II. Questions carrying TWO marks each (answer any five) [5 × 2 = 10] 6. When same amount of electricity is passed through solution of CuSO4 and HCl, 32 mg of copper is obtained in first case. Calculate the volume of hydrogen liberated in the second case. Ans mass of Cu Eq. mass of copper = mass of hydrogen Eq. mass of hydrogen 32 × 10 −3 g × 1 = 10 −3 g 32 22.4 × 10 −3 ∴ volume of hydrogen at STP = dm 3 = 11.2 × 10 −3 dm 3 2

∴ mass of hydrogen =

II IIT-JEE Achiever (Integ.) 2011-13 / Chemistry / Soln. to PU Unit Test - 01

1

7. A weak monoacid base has a pH of 10 at a concentration of 0.01 M. What is percentage ionization of the base? Ans pH = 10 ∴ pOH = 4 ∴ [OH−] = 10−4 M = cα [OH − ] 10 −4 ∴ α= = − 2 × 100 = 1 % c 10 8. Write the equation for rate determining step in the mechanism of hydrolysis of tertiary butyl bromide. Ans +

(CH 3 )3 CBr slow  → (CH 3 )3 C

+ Br −

[1 mark]

Carbocation

[1 mark]

Rate eq. ∴ rate = k[(CH3)3 CBr] 9. Name the reagents used to convert a haloalkane into (a) an alcohol (b) alkanenitrile Ans (a) aq KOH [1] (b) alc KCN

[1]

10. Arrange the following carbocations in the order of their stability and give reason. (i) ter-butyl carbocation (ii) isopropyl carbocation (iii) ethyl carbocation Ans (i) > (ii) > (iii) [1] +I effect of CH3 groups [1] 11. Give two limitations of Arrhenius theory of electrolytic dissociation.

Ans (i) Arrhenius theory fails to account for the conductivity of electrolytes in fused state.

[1]

(ii) It ignores the inter-ionic force of attraction in electrolyte solutions.

[1]

PART - C

III. Questions carrying FIVE marks each (answer any six)

[6 × 5 = 30]

12. (a) Show that pH + pOH = 14 at 298 K. (b) Calculate hydrogen and hydroxyl ion concentration in a solution of pH 1.7. Ans (a) [H+][OH−] = 10−14 at 298 K Taking log and reversal of sign −log[H+] − log[OH−] = 14 , ∴ pH + pOH = 14 (b) pH = 1.7 ∴ [H+] = antilog (−1.7) = 0.02 M 10 −14 ∴ [OH−] = = 5 × 10 −13 M 0.02

II IIT-JEE Achiever (Integ.) 2011-13 / Chemistry / Soln. to PU Unit Test - 01

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[3 + 2]

13. (a) What are the two conditions for an organic compound to show geometrical isomerism? (b) How are the following prepared? (i) Bromoethane from ethanol (ii) Propanenitrile from bromoethane (iii) 2-iodopropane from propene Ans (a) (i) Restricted rotation around carbon-carbon double bond or carbon-carbon single bond. [1 mark] (ii) Two groups attached to the carbon must be different. [1 mark] (b) (i) 3C 2 H 5 − OH + PBr3 → 3C 2 H 5 Br + H 3 PO 3 [1 mark] alc.) (ii) C 2 H 5 Br + KCN ( → C 2 H 5 C ≡ N + KBr

[1 mark]

(iii) CH3 CH CH2 + HI

[1 mark]

CH3 CH CH3 I

14. What is SN2 mechanism? Explain it using hydrolysis of methyl bromide. Ans SN2 : Bimolecular nucleophilic substitution single step / strong nucleophile. Mechanism: H HO + H C Br

H H HO C Br

H

H

• •

[2 + 3]

[1 mark]

H

[4 marks]

HO C H + Br H

Inverted configuration. Walden inversion.

15. (a) What is SN1 mechanism? Explain using hydrolysis of 3° butyl chloride. (b) What is racemic mixture? Give one example.

[3 + 2]

Solution (a) Two step / unimolecular / r = k [RX] Slow → R + + X − (i) R − X  (ii)

R+

carbocation

+ Nu − fast → R − Nu

[1 × 3]

Pr oduct

(b) Equal amount of d and l enantiomers. Inactivity due to external compensation Eg: lactic (±) acid.

[2]

16. (a) Trichloro acetic acid is a stronger acid than acetic acid. Give reason. (b) Give two differences between electromeric and inductive effect. (c) Give an example of a group having −M effect. Solution (a) Cl exerts −I effect [1] Greater the −I effect, stronger the acid [1] (b) Two differences [2] (c) One example [1]

II IIT-JEE Achiever (Integ.) 2011-13 / Chemistry / Soln. to PU Unit Test - 01

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[2 + 2 + 1]

17. (a) A weak acid HA dissociates to an extent of 4.3 % in a 10−3 M solution of it. Calculate its dissociation constant? [2] (b) Define specific conductance. How is it related to molar conductance? [2] (c) Account for the fact that NH3 acts both as a Lewis base and as a Bronsted base. [1] Solution ka k (a) α = ⇒ α2 = a C C 2

 4 .3  −3 ∴ ka = α C =  [1]  × 10  100  = 1.849 × 10−6 [1] (b) It is the conductance of ions present in one meter cube of an electrolyte solution placed between two [1] electrodes of one meter apart. −3 3 c × 10 m [1] µ = k×ν or µ = c (c) Ammonia donates an electron pair – Lewis base [½] Ammonia accepts a proton – Bronsted base [½] 2

18. (a) Give any four assumptions of Arrhenius theory of electrolytic dissociation. (b) What is the [H+] ions in 0.1 M formic acid which is 4.2 % dissociated. Solution (a) Refer Module 2, page 33. (b) [H+] = cα 4 .2 = 0 .1 × = 4.2 × 10 −3 mol dm−3 100

[4] [1]

19. (a) 8g of NaOH is present in 10 dm3 of the solution. Calculate the pH of the solution? (b) Derive Ostwald’s dilution law for a weak electrolyte. (c) Why AlCl3 is a Lewis acid? Solution (a) Mass per 1000 cm3 of NaOH = 0.8 g W 0 .8 Normality = = = 0.02 [½] E 40 [OH−] = 0.02 mol dm−3 [½] − [½] pOH = −log10[OH ] = −log10[0.02] = 1.6990 pH = 14 − pOH = 14 − 1.6990 = 12.3010 [½] (b) Refer Module 2, page 41. α 2c Assumption [1½] Final equation, k a = (1 − α)

[2] [2] [1]

(c) It accepts an electron pair or it is electron deficit.

[1]

*** II IIT-JEE Achiever (Integ.) 2011-13 / Chemistry / Soln. to PU Unit Test - 01

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[½]