Important Questions for CBSE Class 12 Chemistry The Solid State
PREVIOUS YEARS’ QUESTIONS
2015
Very Short Answer Type Questions [1 Mark]
1. What is the formula of a compound in which the element Y forms ccp lattice and atoms of X occupy l/3rd of tetrahedral voids?
Ans.
2. What is the formula of a compound in which the element Y forms ccp lattice and atoms of X occupy 2/3rd of tetrahedral voids?
Ans.
3. What is the formula of a compound in which the element Y forms ccp lattice and atoms of X occupy 2/3rd of octahedral voids?
Ans.
4. Name the type of crystal defect which is produced when NaCl crystal is doped with MgClj.
Ans. Cation vacancy defect or Non-stoichiometric defect or impurity defects.
5. What is the formula of a compound in which the element ‘Y’ form hep lattice and atoms of ‘X’ occupy 1/3 r“ of octahedral voids.
Ans.
6. Why type of stoichiometric defect is shown by AgCl?
Ans. AgCl shows Frenkel defects.
Short Answer Type Questions [I] [2 Marks}
7. The compound CuCl [Formula mass = 99 g mol-1] has FCC structure like ZnS. Its density is 3.4 g cm-3. What is the length of the edge of unit cell? [NA = 6.02 X 1023 mol-1]
Ans.
Short Answer Type Questions [II] [3 Marks]
8. An element with density 10 g cm-3forms a cubic cell with edge length of 3 X 10-8 cm. What is the nature of cubic unit cell if the atomic mass of the element is 81 g mol?
Answer:
9. Define the following:
(i) Primitive unit cell (ii) Schottky defect (iii)Ferromagnetism.
Answer: (i) The cell in which particles are at corners only.
(ii) It is due to missing of cations and anions from their positions.
(iii) The magnetic moments are permanently aligned in same direction. It has permanent high magnetic moment.
10. Define the following:
(i) F-centre (ii) p-type semiconductor
(iii) Ferrimagnetism.
Answer: (i) Anion vacancies occupied by free electrons as in alkali metal halides.
(ii) Si and Ge doped with trivalent atoms creating electron vacancies, +ve hole increases electrical conductance.
(iii) Magnetic dipole’s are aligned parallel and antiparallel but unequal in number leading to small magnetic moment.
11. Examine the given defective crystal:
Answer the following questions:
(i) Is the above defect stoichiometric or non-stoichiometric?
(ii) Write the term used for this type of defect?
(iii) Why do silver halides show this type of defect?
Answer: (i) Stoichiometric (ii) Frenkel defect
(iii) It is because Ag+ and X- differ appreciably in their size where X represents any halide ion.
12. Define the following?
(i) Crystalline solids (i) Frenkel defects (iii) n-type semiconductor.
Answer: (i) Those solids which have 3-D arrangement of atoms, ions or molecules are called crystalline solids, e.g. diamond.
(ii) This is due to shifting of an ion from its normal position to an interstitial, in the crystal lattice, thus causing a vacancy in the original position. The overall density is not affected by this change.
(iii) When group 14 elements like Si, Ge are doped with group 15 elements like As, Sb, we get n-type semiconductors.
13. Define the following:
(i) Schottky defect (ii) Frenkel defects (iii) F-centre
Answer: (i) Refer Ans. to Q.9 (ii).
(ii) Refer Ans. to Q.12 (ii).
(iii) Refer Ans. to Q.10 (i).
14. An element with molar mass 27 g mol-1 forms a cubic unit cell with edge length 4.05 X 10-8cm. If its density is 2.7 g cm-3, what is the nature of the cubic unit cell?
Answer:
15. Examine the given defective crystal:
Answer the following questions:
(i) Is the above defect stoichiometric or non-stoichiometric?
(ii) Write the term used for this type of defect. Give an example of the compound which shows this type of defect. .
(iii) How does this defect affect the density of the crystal?
Answer: (i) It is stoichiometric defect.
(ii) Schottky defect. NaCl shows this type of defect.
(iii) Density of crystal decreases.
16. An element X (molar mass = 60 g mol-1) has a density of 6.23 g cm-3. Identify the type of cubic unit cell, if the edge length of the unit cell is 4 X 10-8cm.
Answer:
17. Examine the given defective crystal:
Answer the following questions:
(i) Is the above defect stoichiometric or non-stoichiometric?
(ii) Write the term used for the electron occupied site.
(iii) Give an example of the compound which shows this type of defect.
Answer: (i) Non-stoichiometric
(ii) F-centre.
(iii) When LiCl is heated with lithium vapours, it shows this type of defect.
2014
Very Short Answer Type Questions [1 Mark]
18. Why are crystalline solids anisotropic?
Answer: It is due to different alignment of particles in different directions.
19. Express the relationship between atomic radius (r) and the edge length (a) in the bcc unit cell.
Answer:
20. Express the relationship between atomic radius(r) and the edge length (a) of the fee unit cell.
Answer:
Short Answer Type Questions [I] [2 Marks]
21. An element with density 11.2 g cm-3 forms an fee lattice with edge length of 4 X 10-8cm. Calculate the atomic mass of the element. (Given: NA = 6.022 X 1023 mol-1)
Answer:
22. Examine the given defective crystal.
Answer the following questions:
(i) What type of stoichiometric defect is shown by the crystal?
(ii) How is the density of the crystal affected by this defect?
(iii) What type of ionic substances show such defect?
Answer: (i) Schottky defect.
(ii) Density of crystal decreases.
(iii) Those ionic substances in which cations and anions are of similar size, e.g. NaCl and NaBr.
23. An element with density 2.8 g cm-3 forms a fee unit cell with edge length 4 X 10-8 cm. Calculate the molar mass of the element. (Given: N. = 6.022 X 1023 mol-1).
Answer:
24. (i) What type of non-stoichiometric point defect is responsible for the pink colour of LiCl?
(ii) What type of stoichiometric defect is shown by NaCl?
Answer:
(i) Metal excess defect.
(ii) Schottky defect.
25. How will you distinguish between the following pairs of terms:
(i) Tetrahedral and octahedral voids
(ii) Crystal lattice and unit cell
Answer:
(i) The void between three spheres in direct contact and capped by fourth sphere is called tetrahedral void. Its coordination number is 4.
The void between three spheres in direct contact capped by three more spheres is called octahedral void. Its coordination number is 6.
(ii) Crystal lattice: It is 3-D arrangement of atoms, ions or molecules.
Unit cell: It is a portion of crystal lattice which on repetition gives crystal lattice.
26. (i) Write the type of magnetism observed when the magnetic moments are oppositely aligned and cancel out each other.
(ii) Which stoichiometric defect does not change the density of the crystal?
Answer:
(i) Antiferromagnetism
(ii) Frenkel defect.
27. (i) Write the type of magnetism observed when the magnetic moments are aligned in parallel and anti-parallel directions in unequal numbers.
(ii) Which stoichiometric defect decreases the density of the crystal?
Answer:
(i) Ferrimagnetism (ii) Schottky defects
Short Answer Type Questions [II] [3 Marks]
28. (i) What type of stoichiometric defect is shown by KC1 and why?
(ii) What type of semiconductor is formed when silicon is doped with As?
(iii)Which one of the following is an example of molecular solid: C02 or Si02
(iv) What type of substances would make better magnets, ferromagnetic or ferrimagnetic?
Answer:
(i) KC1 shows Schottky defects because K+ and Cl- do not differ appreciably in their size.
(ii) n-type semiconductor.
(iii)C02 is an example of molecular solid.
(iv)Ferromagnetic substances are used for making permanent magnets.
2013
Very Short Answer Type Questions [1 Mark]
29. How many atoms can be assigned to its unit cell if an element forms
(i) a body-centred cubic cell, and (ii) a face-centred cubic cell?
Answer:
(i) Z = 2 for bcc element (ii) Z = 4 for fee element.
30. Why is glass considered a supercooled liquid?
Answer: It is because glass windows are thicker at the bottom and thinner at the top. It shows that glass can flow, that is why it is called supercooled liquid.
31. What types of solids are electrical conductors, malleable and ductile?
Answer: Metallic solids are electrical conductors, malleable and ductile.
32. What is the two-dimensional coordination number of a molecule in a square
close packed layer?
Answer: Four
33. Which type of ionic substances show Schottky defect in solids?
Answer: Those ionic solids in which cation and anion are of almost similar sizes, e.g. NaCl.
34. On heating a crystal of KC1 in potassium vapour, the crystal starts exhibiting
a violet colour. What is this due to?
Answer: It is due to loss of electron by potassium atom to form K+ ions which attract Cl- ions and position of Cl~ ions is occupied by electron forming F-centre which absorbs light from visible region and radiates violet colour.
35. What type of substances would make better permanent magnets, Ferromagnetic or Ferrimagnetic?
Answer: Ferromagnetic substances would make better permanent magnets.
36. What type of stoichiometric defect is shown by AgCl? [Delhi]
Answer: Refer Ans. to Q.6.
37. How many atoms constitute one unit cell of a face-centred cubic crystal?
Answer: Four.
Short Answer Type Questions [I] [2 Marks]
38. (a) Why does Li Cl acquire pink colour when heated in Li vapours? (b) A solid with cubic crystal is made of two elements ‘P’ and ‘Q’ Atoms of ‘Q’ are at the comers of the cube and ‘P’ at body centre. What is the formula of compound?
Answer: (a) LiCl when heated with lithium vapours, Li+ become in excess which attract Cl- and the vacant position of anion is occupied by electron forming F-centre which absorbs light from visible region and radiates pink colour.
(b) No. of Q atoms = 8 x 1/8 = 1, No. of P atoms=1
.•. Formula of compound is PQ.
39. Account for the following:
(i) Schottky defects lower the density of related solids.
(ii) Conductivity of silicon increases on doping it with phosphorus.
Answer: (i) In Schottky defects, large number of cations and anions are missing which lead to lowering in density of crystal lattice.
(ii) Phosphorus has five electrons, whereas silicon has four, one excess electron becomes free to move which leads to increase in conductivity.
40. Niobium crystallises in body-centred cubic structure. If density is 8.55 g cm-3, calculate atomic radius of niobium using its atomic mass 93 u.
Answer:
41.(a) What change occurs when AgCl is doped with CdCl2?
(b) What type of semiconductor is produced when silicon is doped with boron?
Answer: (a) Its conductance increases due to movement of Ag+ ions.
(b) jb-type semiconductor.
42. Aluminium crystallizes in an fee structure. Atomic radius of the metal is 125 pm. What is the length of the side of the unit cell of the metal?
Answer:
Short Answer Type Questions [II] [3 Marks]
43. (a) What type of semiconductor is obtained when silicon is doped with boron?
(b) What type of magnetism is shown in the following alignment of magnetic moments?
↑↑↑↑↑↑↑↑↑↑↑↑
(c) What type of point defect is produced when AgCl is doped with CdCl2?
Answer: (a) jb-type semiconductor
(b) Ferromagnetism
(c) Impurity defects
44. Iron(II) oxide has a cubic structure and each side of unit cell is 5 A. If density of the oxide is 4 g cm-3, calculate the number of Fe+2and O-2ions present in each unit cell. [Atomic mass of Fe = 56 u, O = 16 u, NA = 6.023 X 1023 mol-1]
Answer:
45. An element occurs in bcc structure. It has a cell edge length of250 pm. Calculate the molar mass if its density is 8.0 g cm-3. Also calculate the radius of an atom of this element.
Answer:
46. An element with molar mass 27 g mol-1 forms a cubic unit cell with edge length 4.05 X 10-8 cm. If its density is 2.7 g cm-3, what is the nature of the cubic unit cell?
Answer: Refer Ans. to Q. 14.
2012
Very Short Answer Type Questions [1 Mark]
47. What type of stoichiometric defect is shown by AgBr and Agl?
Answer: Frenkel defects are shown by AgBr and Agl. AgBr shows Schottky defect also.
48. What type of defect can arise when a solid is heated?
Answer:When a solid is heated, it can have vacancy defect (Schottky defect).
49. What is meant by ‘doping’ in a semiconductor?
Answer: The introduction of a small amount of impurity in a semiconductor is called doping.
50. What are n-type semiconductors?
Answer: Refer Ans. to Q.12 (iii).
51. Write a point of distinction between a metallic solid and an ionic solid other than metallic lustre.
Answer: Metallic solids can conduct electricity due to movement of electrons in solid state, whereas ionic solids conduct electricity in molten state or in aqueous solution only due to the movement of ions.
52. How may the conductivity of an intrinsic semiconductor be increased?
Answer: It can be increased by adding appropriate amount of suitable impurity. This process is called doping.
53. What is meant by the term ‘forbidden zone’ in reference to band theory of solids?
Answer: The gap between valence band and conduction band in the case of insulator is very large. This zone is called forbidden zone.
54. Define paramagnetism with an example.
Answer: The property due to which substances are attracted towards magnets. They have permanent magnetic moment due to unpaired electrons, e.g. Cu+2, 02, Fe-2.
55. Which stoichiometric defect in crystals increases the density of a solid?
Answer: Interstitial defect increases the density of a solid.
Short Answer Type Questions [II] [3 Marks]
56. Tungsten crystallizes in body-centred cubic unit cell. If the edge of the unit cell is 316.5 pm, what is the radius of tungsten atom?
Answer:
57. Iron has a body-centred cubic unit cell with a cell edge of 286.65 pm. The density of iron is 7.874 g cm-3. Use this information to calculate Avogadro’s number. (At mass of iron = 55.845 u)
Answer:
58. Silver crystallises in face-centred cubic (fee) unit cell. If the radius of silver atom is 145 pm, what is the length of each side of the unit cell?
Answer:
59. Copper crystallises with face-centred cubic unit cell. If the radius of copper atom is 127.8 pm, calculate the density of copper metal. (Atomic mass of Cu = 63.55 u and Avogadro’s number (NA) = 6.02 X 1023 mol-1)
Answer:
60. (a)What are intrinsic semiconductors? Give an example.
(b) What is the distance between Na+and Cl– ions in NaCl crystal if its density is 2.165 g cm-3. [Atomic mass of Na = 23 u, Cl = 35.5 u, Avogadro’s Number = 6.023 X 1023}
Answer:
2011
Very Short Answer Type Questions [1 Mark]
61. ‘Crystalline solids are anisotropic in nature.’ What does this statement mean?
Answer: It means crystalline substances show different optical and electrical properties in different directions, i.e. anisotropic in nature.
62. What is meant by an ‘intrinsic semiconductor’?
Answer: A semiconductor which is used in pure form is called ‘intrinsic semiconductor’
Short Answer Type Questions [I] [2 Marks]
63. Explain the following terms with suitable examples of each: ‘
(i) Ferromagnetism
(ii) Antiferromagnetism
Answer: (i) Ferromagnetism: When magnetic moments are aligned in the same direction, it gives rise to ferromagnetism, e.g. iron, cobalt and nickel show ferromagnetism.
Schematic alignments of magnetic moments in ferromagnetism
(ii) Antiferromagnetism: When magnetic moments are aligned in such a way that net magnetic moment is zero, then magnetism is called anti ferromagnetism, e.g. MnO shows antiferromagnetism.
Schematic alignments of magnetic moments in antiferromagnetism
64. Calculate the packing efficiency of a metal crystal for simple cubic lattice.
Answer:
65. An alloy of gold and cadmium crystallises with a cubic structure in which gold atoms occupy the corners and cadmium atoms fit into the face centre. Assign formula for this alloy.
Answer: As there are 8 corners in a cube and gold atoms are present at the corners of the cube. ‘
Thus, 8 gold atoms at the eight corners contribute 8 X 1/8= 1 atom. [Contribution of an atom at the corner per unit cell is 1/8]
As there are 6 face centres in a cube and cadmium atoms fit into six face centresof the cube.
Thus, 6 cadmium atoms at the six faces contribute 6 x 1/2 = 3. [Contribution per unit cell of an atom at face centre is 1/2.]
Thus, formula of this alloy is AuCd3.
66. In terms of band theory, what is the difference
(i) between a conductor and a semiconductor?
(ii) between a conductor and an insulator?
Answer: (i) In a semiconductor, there is small energy gap between valence band and conduction band, whereas in a conductor, either there is partially filled valence band or there is overlapping between valence band and conduction band.
(ii) In a conductor, there is overlapping between conduction band and valence band. In an insulator, there is a large energy gap between conduction band and valence band.
67. Explain how you can determine the atomic mass of an unknown metal if you know its mass density and the dimensions and type of unit cell of its crystal.
Answer:
68. Define the following terms in relation to crystalline solids:
(i) Unit cell (ii) Coordination number
Give one example in each case.
Answer: (i) Unit cell: It is the smallest repeated unit in a crystal lattice, e.g. in sodium chloride and caesium chloride, the unit cell is a cube. We can generate the full lattice by repeatedly moving the unit cell in the direction of its edges by a distance equal to the cell edge.
(ii) Coordination number: It is the number of atoms surrounding a central metal atom, e.g. coordination number of central sphere is 8 in body-centred cubic structures.
Short Answer Type Questions [II] [3 Marksl
69. The density of lead is 11.35 g cm-3 and the metal crystallizes with fee unit cell. Estimate the radius of lead atom. (At Mass of lead = 207 g mol-1 and NA = 6.02 X 1023mol-1
Answer:
70. Aluminium crystallises in a cubic close-packed structure. Radius of the atom in the metal is 125 pm.
(i) What is the length of the side of the unit cell?
(it) How many unit cells are there in 1 cm3 of aluminium?
Answer:
71. Silver crystallizes in face-centred cubic unit cell. Each side of the unit cell has a length of 400 pm. Calculate the radius of the silver atom. (Assume the atoms just touch each other on the diagonal across the face of the unit cell. That is each face atom is touching the four corner atoms.)
Answer:
2010
Very Short Answer Type Questions [1 Mark]
72. Write a feature which will distinguish a metallic solid from an ionic solid.
Answer: Metallic solids conduct electricity in solid state through electrons, while ionic solids conduct electricity in molten state or in aqueous solution through ions. Metals are malleable and ductile, whereas ionic solids are hard and brittle.
73. Write a distinguishing feature of metallic solids.
Answer: Metals are malleable and ductile. They conduct electricity.
74. What type of interactions hold the molecules together in a polar molecular solid?
Answer: Dipole-dipole interactions, dispersion forces and van der Waals’ forces of attraction hold the molecules together in a polar molecular solid.
75. What type of semiconductor is obtained when silicon is doped with arsenic?
Answer: n-type semiconductor is obtained when silicon is doped with arsenip
Answer: Schottky defect decrease the density of the solid.
77. Give an example of an ionic compound which shows Frenkel defect.
Answer: AgCl shows Frenkel defect.
78. What is the number of atoms in a unit cell of a simple cubic crystal?
Answer: The number of atoms in a unit cell of a simple cubic crystal is one [8*1/8=1].
79. Which point defect in crystals of a solid does not change the density of the solid?
Answer: Frenkel defect.
80. Write a point of distinction between a metallic solid and an ionic solid other than metallic lustre.
Answer: Refer Ans. to Q.51.
Short Answer Type Questions [I] [2 Marks]
81. How are the following properties of crystals affected by Schottky and Frenkel defects?
(i) Density
(ii) Electrical conductivity
Answer: (i) Density remains the same in Frenkel defect, whereas Schottky defect leads to lowering of density.
(ii) Electrical conductivity increases in both Frenkel and Schottky defects.
82. Copper crystallizes into an fee lattice with edge length 3.61 X 10-8 cm. Calculate the density of copper. [Given: Cu = 63.5 g mol-1, NA = 6.022 X 1023mol-1]
Answer:
Short Answer Type Questions [II] [3 Marks]
83. The well known mineral fluorite is chemically calcium fluoride. It is known that in one unit cell of this mineral there are 4 Ca2+ ions and 8 F–. ions and that Ca2+ ions are arranged in a fee lattice. The F- ions fill all tetrahedral holes in the face centred cubic lattice of Ca2+ ions. The edge of the unit cell is 5.46 X 10-8 cm in lengtli. The density of the solid is 3.18 g cm-3. Use this information to calculate Avogadro’s number. (Molar mass of CaF2 = 78.08 g mol-1)
Answer:
84. The density of copper metal is 8.95 g cm-3. If the radius of copper atom is 127.8 pm, is the copper unit cell a simple cubic, a body-centred cubic or a face-centred cubic structure? (Given: At. mass of Cu = 63.54 g mol-1 and NA = 6.02 X 1023 mol-1).
Answer:
85. Silver crystallises in a fee lattice. If the edge length of the unit cell is 4.07 X 108 cm and density of silver is 10.5 g cm-3, calculate the atomic mass of silver.(NA = 6.02 X 1023 atoms mol-1)
Answer:
86. Copper crystallises in face-centred cubic lattice and has density of 8.930 g cm-3 at 293 K. Calculate the radius of copper atom. [Atomic mass of Cu = 63.55 u, NA = 6.02 X 1023 mol-1]
Answer:
87. Iron has a body-centred cubic unit cell with a cell edge of 286.65 pm. The density of iron is 7.874 g cm-3. Use this information to calculate Avogadro’s number. (At mass of iron = 55.845 u)
Answer: Refer Ans. to Q.57.
88. Silver crystallizes in face-centred cubic unit cell. Each side of the unit cell has a length of 400 pm. Calculate the radius of the silver atom. (Assume the atoms just touch each other on the diagonal across the face of the unit cell. That is each face atom is touching the four corner atoms.)
Answer: Refer Ans. to Q.71.
2009
Very Short Answer Type Questions [1 Mark]
89. Which point defect in its crystal units alters the density of a solid?
Answer: Schottky defect alters the density of a solid.
90. How many atoms can be assigned to its unit cell if an element forms (i) a body-
centred cubic cell, and (ii) a face-centred cubic cell?
Answer: (i) 2 (ii) 4
91. How many atoms constitute one unit cell of a face-centred cubic crystal?
Answer: Four.
92. ‘Crystalline solids are anisotropic in nature.’ What does this statement mean?
Answer: Refer Ans. to Q.61.
93. Which point defect in crystals of a solid decreases the density of the solid?
Answer: Refer Ans. to Q.76.
Short Answer Type Questions [II] [3 Marks]
94. Silver crystallizes in face-centred cubic unit cell. Each side of the unit cell has
a length of 400 pm. Calculate the radius of the silver atom. (Assume the atoms just touch each other on the diagonal across the face of the unit cell. That is each face atom is touching the four corner atoms.)
Answer: Refer Ans. to Q.71.
95. Iron has a body-centred cubic unit cell with a cell edge of286.65 pm. The density
of iron is 7.874 g cm-3. Use this information to calculate Avogadro’s number. (At mass of iron = 55.845 u)
Answer: Refer Ans. to Q.57.