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Board Paper of Class 12 2019 Chemistry - Solutions

All questions are compulsory
Question 1 is of 20 marks having four sub parts, all of which are compulsory.
Question numbers 2 to 8 carry 2 marks each, with two questions having internal choice.
Question numbers 9 to 15 carry 3 marks each, with two questions having an internal choice.

Question numbers 16 to 18 carry 5 marks each, with an internal choice.
All working, including rough work, should be done on the same sheet as, and adjacent to the rest of the answer.

The intended marks for questions or parts of questions are given in brackets [ ].
Balanced equations must be given wherever possible and diagrams where they are helpful.
When solving numerical problems, all essential working must be shown.
In working out problems, use the following data:

Gas constant R = 1.987 cal deg–1 mol–1 = 8.314 JK–1 mol–1 = 0.0821 dm3 atm K–1mol–1
1 l atm = 1 dm3 atm = 101.3 J. 1 Faraday = 96500 coulombs.
Avogadro’s number = 6.023 ×1023.

  • Question 1
    (a)  Fill in the blanks by choosing the appropriate word/words from those given in the brackets:
    (more than, primary, cathode, Lucas reagent, two, four, less than, Grignard’s reagent, tertiary, anode, zero, equal to, three)
    (i) The elevation of boiling point of 0·5 M K2SO4 solution is ___________that of 0·5 M urea solution. The elevation of boiling point of 0·5 M KCl solution is __________ that of 0·5 M K2SO4 solution.
    (ii) A mixture of conc. HCl and anhydrous ZnCl2 is called __________ which shows maximum reactivity with _________ alcohol.
    (iii) In electrolytic refining the impure metal is made __________ while a thin sheet of pure metal is used as _________.
    (iv) When the concentration of a reactant of first order reaction is doubled, the rate of reaction becomes _________ times, but for a __________ order reaction, the rate of reaction remains the same.

    (b) Select the correct alternative from the choices given:
    (i) The cell reaction is spontaneous or feasible when emf of the cell is:
    (1) negative
    (2) positive
    (3) zero
    (4) either positive or negative

    (ii) Which, among the following polymers, is a polyester:
    (1) melamine
    (2) bakelite
    (3) terylene
    (4) polythene

    (iii) The correct order of increasing acidic strength of the oxoacids of chlorine is:
    (1) HClO3 < HClO4 < HClO2 < HClO
    (2) HClO < HClO2 < HClO3 < HClO4
    (3) HClO2 < HClO < HClO4 < HClO3
    (4) HClO3 < HClO4 < HClO < HClO2

    (iv) A catalyst is a substance which:
    (1) changes the equilibrium constant of the reaction.
    (2) increases the equilibrium constant of the reaction.
    (3) supplies energy to the reaction.
    (4) shortens the time to reach equilibrium.

    (c) Match the following:
     (i) Diazotisation  (a) Anisotropic
     (ii) Crystalline solid  (b) Reimer-Tiemann reaction
     (iii) Phenol  (c) Diphenyl
     (iv) Fittig reaction  (d) Aniline


    (d) (i) (1) Which trivalent ion has maximum size in the Lanthanoid series i.e. Lanthanum ion (La3+) to Lutetium ion (Lu3+)?
    (at. no. of Lanthanum = 57 and Lutetium = 71)
    (2) Explain why Cu2+ is paramagnetic but Cu+ is diamagnetic.(at. no. of Cu = 29)

    (ii) When a coordination compound CoCl3.6NH3 is mixed with AgNO3, three moles of AgCl are precipitated per mole of the compound. Write the structural formula and IUPAC name of the coordination compound.

    (iii) Calculate the boiling point of urea solution when 6 g of urea is dissolved in 200 g of water.
    (Kb for water = 0·52 K kg mol–1, boiling point of pure water = 373 K, mol. wt. of urea = 60)

    (iv) Identify the compounds A, B, C and D in the given reaction:
    HCCHHg2+/H2SO4H2OA K2Cr2O7+H2SO4O B CaOH2Cdry distillation heatD

  • Question 2
    (a) For the reaction A + B → C + D, the initial rate for different reactions and initial concentration of reactants are given below:
    S. No. Initial Conc.  Initial rate 
    (mole L–1 sec–1)
    [A] mole L–1  [B] mole L–1
    1 1.0 1.0 2 × 10–3
    2 2.0 1.0 4 × 10–3
    3 4.0 1.0 8 × 10–3
    4 1.0 2.0 2 × 10–3
    5 1.0 4.0 2 × 10–3
    (i) What is the overall order of reaction?
    (ii) Write the rate law equation.


    (b) 25% of a first order reaction is completed in 30 minutes. Calculate the time taken in minutes for the reaction to go to 90% completion. VIEW SOLUTION

  • Question 3
    (i) Name the type of drug which lowers the body temperature in high fever condition.
    (ii) What are tranquilizers? Give one example of a tranquilizer. VIEW SOLUTION

  • Question 4
    Write the balanced chemical equation for each of the following:
    (i) Chlorobenzene treated with ammonia in the presence of Cu2O at 475 K and 60 atm.
    (ii) Ethyl chloride treated with alcoholic potassium hydroxide. VIEW SOLUTION

  • Question 5
    (i) Name the monomer and the type of polymerisation that takes place when PTFE is formed.
    (ii) Name the monomers of nylon 6, 6. VIEW SOLUTION

  • Question 6
    Name two water soluble vitamins and the diseases caused by their deficiency in the diet of an individual. VIEW SOLUTION

  • Question 7
    (a) How will you obtain the following (give balanced chemical equations):
    (i) Benzene from phenol.
    (ii) Iodoform from ethanol.


    (b) How will you obtain the following (give balanced chemical equations):
    (i) Salicylaldehyde from phenol.
    (ii) Propan–2–ol from Grignard’s reagent.

  • Question 8
    Show that for a first order reaction the time required to complete 75% of reaction is about 2 times more than that required to complete 50% of the reaction. VIEW SOLUTION

  • Question 9
    (a) When 0·4g of acetic acid is dissolved in 40g of benzene, the freezing point of the solution is lowered by 0·45K.
    Calculate the degree of association of acetic acid. Acetic acid forms dimer when dissolved in benzene.
    (Kf for benzene = 5·12 K kg mol–1, at. wt. C = 12, H = 1, O = 16)


    (b) A solution is prepared by dissolving 9·25g of non–volatile solute in 450 ml of water. It has an osmotic pressure of 350 mm of Hg at 27ºC.
    Assuming the solute is non–electrolyte, determine its molecular mass.
    (R = 0·0821 lit atm K–1 mol–1)

  • Question 10
    An element occurs in body centered cubic structure. Its density is 8·0 g/cm3. If the cell edge is 250 pm, calculate the atomic mass of an atom of this element.
    (NA = 6·023 ×1023) VIEW SOLUTION

  • Question 11
    Describe the role of the following:
    (i) Cryolite in the extraction of aluminium from pure alumina.
    (ii) NaCN in the extraction of silver from a silver ore.
    (iii) Coke in the extraction of iron from its oxides. VIEW SOLUTION

  • Question 12
    (i) Write the IUPAC names of the following:
    (1) K3[Fe(C2O4)3]
    (2) [Co(NH3)5Cl]SO4

    (ii) [Fe(CN)6]4– is a coordination complex ion.
    (1) Calculate the oxidation number of iron in the complex.
    (2) Is the complex ion diamagnetic or paramagnetic?
    (3) What is the hybridisation state of the central metal atom?
    (4) Write the IUPAC name of the complex ion.

  • Question 13
    (a) Explain why:
    (i) Transition elements form alloys.
    (ii) Zn2+ salts are white whereas Cu2+ salts are coloured.
    (iii) Transition metals and their compounds act as catalyst.


    (b) Complete and balance the following chemical equations.
    (i) KMnO4 + H2SO4 + H2C2O4 → ______ + ______ + ______ + ______
    (ii) K2Cr2O7 + H2SO4 + KI → ______ + ______ + ______ + ______
    (iii) K2Cr2O7 + H2SO4 + FeSO4→  ______ + ______ + ______ + ______

  • Question 14
    Give balanced equations for the following:
    (i) Aniline is treated with bromine water.
    (ii) Ethylamine is heated with chloroform and alcoholic solution of potassium hydroxide.
    (iii) Benzene diazonium chloride is treated with ice cold solution of aniline in acidic medium. VIEW SOLUTION

  • Question 15
    Define the following terms with suitable examples:
    (i) Peptisation
    (ii) Electrophoresis
    (iii) Dialysis VIEW SOLUTION

  • Question 16
    (a) (i) Calculate the mass of silver deposited at cathode when a current of 2 amperes is passed through a solution of AgNO3 for 15 minutes.
    (at. wt. of Ag = 108, 1 F = 96,500 C)
    (ii) Calculate the emf and ΔG for the cell reaction at 298 K
    MgsMg2+0.1MCu2+0.01MCusGiven Ecell0=2.71 V1F=96,500 C


    (b) (i) Define the following terms:
    (1) Specific conductance
    (2) Kohlrausch’s Law

    (ii) The resistance of a conductivity cell containing 0·001 M KCl solution at 298 K is 1500 ohm.
    What is the cell constant and molar conductivity of 0·001 M KCl solution, if the conductivity of this solution is 0·146 × 10–3 ohm–1 cm–1 at 298 K?

  • Question 17
    (a) (i) Explain why:
    (1) Fluorine has lower electron affinity than chlorine.
    (2) Red phosphorus is less reactive than white phosphorous.
    (3) Ozone acts as a powerful oxidising agent.
    (ii) Draw the structures of the following:
    (1) XeF6
    (2) IF7


    (b) (i) Explain why:
    (1) Interhalogen compounds are more reactive than the related elemental halogens.
    (2) Sulphur exhibits tendency for catenation but oxygen does not.
    (3) On being slowly passed through water, PH3 forms bubbles but NH3 dissolves.
    (ii) Complete and balance the following reactions:
    (1) P4 + H2SO4 → ______ + ______ + ______
    (2) Ag + HNO3dilute → ______ + ______ + ______

  • Question 18
    (a) (i) Give balanced chemical equations for the following reactions:
    (1) Acetaldehyde reacts with hydrogen cyanide.
    (2) Acetone reacts with phenyl hydrazine.
    (3) Acetic acid is treated with ethanol and a drop of conc. H2SO4.
    (ii) Give one chemical test each to distinguish between the following pairs of compounds:
    (1) Acetone and benzaldehyde.
    (2) Phenol and benzoic acid.


    (b) (i) Write chemical equations to illustrate the following name reactions:
    (1) Aldol condensation.
    (2) Cannizzaro’s reaction.
    (3) Benzoin condensation.

    (ii) Identify the compounds A and B in the given reactions:
    (2) CH3COCH3 HNO3conc.O A PCl5 B
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