SIQ Class 09 || Ch. 03 || Atoms and Molecules

SIQ Class 09 || Ch. 03 || Atoms and Molecules

Some Important Questions

CHAPTER 3

Atoms and Molecules

ONE MARK QUESTIONS

1. State the law of conservation of mass.

Ans : This law states that the mass can neither be created nor destroyed in a chemical reaction. That is ‘bass of reactants is always equal to mass of products.

2. Who was the first scientist to give the concept of formation of compounds?

Ans : Antoine L. Lavoisier gave the concept of formation of 

compounds.

3. Who stated the Law of Constant Proportion? 

Ans : Joseph Louis Proust stated the Law of Constant Proportion.

4. What is the building block of all matter?

Ans : Atom is the building block of all matter.

5. What is the measuring unit of atomic radius?

Ans : 

Nanometre (nm) is the measuring unit of atomic radius.

6. Name the instrument which produces image of the surface of element that shows atoms.

Ans : Scanning tunnelling microscope.

7. What is the full form of IUPAC?

Ans : International Union of Pure and Applied Chemistry.

8. Give the derivation source of symbol of sodium (Na).

Ans : The symbol of ‘Na’ for sodium is derived from its Latin name ‘Natrium’.

9. Give the full form of amu? 

Ans : The full from of amu is atomic mass unit.

10. What is the latest short form of atomic mass unit?

Ans : The latest short form of atomic mass unit is u, according to IUPAC.

11. Name any two monatomic atoms. 

Ans : Sodium, Aluminium.

12. What do we get if 3 atoms of oxygen unite into a molecule, instead of usual 2 ?

Ans : We get (O3) ozone.

13. Give difference between 2H and H2.

Ans : 2H indicates 2 atoms of hydrogen and H2 indicates one 

molecule of hydrogen.

14. Give two examples of triatomic molecules. 

Ans : Carbon dioxide (CO2) and water (H2O).

15. Define valency.

Ans : The combining power of an element to attain the noble gas configuration is called valency. Or, it is defined as number of electrons lost or gained by an atom to acquire noble gas configuration.

16. Who introduced the word ‘Mole’? 

Ans : ‘Wilhelm Ostwald’ introduced the word ‘Mole’.

17. What is Avogadro Constant?

Ans : The number of particles present in one mole of any substance is fixed with a value of 6.022 × 1023.

18. How does an atom exist?

Ans : Atom exists in the form of atom, molecule or ions.

19. Give Latin name of Silver. 

Ans : Latin name of Silver is ‘Argentum’.

20. What is the symbol of the element of molybdenum? 

Ans : ‘Mo’ is the symbol of the element of molybdenum.

21. What is ion?

Ans : An ion is a charged particle. It can be positive or negative.

22. What do you mean by symbols of elements?

Ans : Each element is represented by a letter or group of two letters to write the chemical reactions conveniently. It is called symbol.

23. Give the symbol of copper, silver, gold, oxygen, zinc.

Ans : 

Copper - Cu

Silver - Ag

Gold - Au

Oxygen - O

Zinc - Zn

24. What is the difference between an atom and molecule?

Ans : An atom is the smallest particle of an element which 

may or may not have independent existence. For example : Na, Al, Fe, etc.

Molecule is the smallest particle of the element or compound which can exist independently. For example : O2, H2, N2, etc.

25. Name two elements which have same atomic number.

Ans : Two elements cannot have the same atomic number.

26. An element has 8 electrons in its valence shell. What is its general name?

Ans : Noble gas.

27. Write the value of charge of electron. 

Ans : 1.6 × 10–19 coulomb.

28. Name the gas which gives the lightest positively charged particle.

Ans : Hydrogen.

29. How did the scientist lay the foundation of chemical sciences? Name the scientist.

Ans : Antoine Laurent Lavoisier laid the foundation of chemical science by establishing two important laws of chemical combination.

30. Define law of conservation of mass.

Ans : Law of conservation of mass states that, ‘Mass is neither created nor destroyed in a chemical reaction.’

31. Define law of constant proportion.

Ans : Law of constant proportion states that, ‘In a pure chemical substance, the elements are always present in definite proportions by mass.’

32. Which organisation approves the names of elements all over the world?

Ans : International Union of Pure and Applied Chemistry (IUPAC).

33. Write the symbols of tungsten and iron. 

Ans : (i) Tungsten (W) and(ii) Iron (Fe)

34. Name the element which is used as the reference for atomic mass.

Ans : Carbon.

35. Name two atoms which exist as independent atoms. 

Ans : Noble gases such as argon (Ar) and helium (He) exist as independent atoms

36. What is the number of electrons in Mg atom and Mg2+

ion?

Ans :  Mg = 12e–

 Mg2+ = 10e–

37. Write atomicity of the following :(i) Sulphur, (ii) Phosphorus

Ans : (i) Polyatomic, (ii) Tetra atomic.

38. Define atomicity.

Ans : The number of atoms present in one molecule of an 

element or a compound is known as its atomicity.

39. What are polyatomic ions? Give two examples.

Ans : A group of atoms having a charge is known as polyatomic ion.Examples : (NH4)+ (SO4)2–

40. What is the atomicity of argon? 

Ans : Mono atomic.

41. Give one relevant reason, why scientists choose 1/16thof the mass of an atom of naturally occurring oxygen as the atomic mass unit?

Ans : Initially, 1/16th of the mass of naturally occurring oxygen was taken as the atomic mass unit because this unit gave masses of most of the elements as whole numbers.

42. State the number of hydrogen atoms in 1 g of hydrogen.

Ans : One gram of hydrogen = One mole = 6.022 × 1023 atoms

43. What is molar mass? What are its units?

Ans : 

The mass of one mole of a substance is called its molar mass. Its unit is gram per mole (gmol–1).

44. The relative atomic mass of oxygen atom is 16. Explain its meaning.

Ans : 

The relative atomic mass of an atom is the average masses of the atom, as compared to 1/12th

the mass of one carbon-12 atom.

45. Distinguish between molecular mass and molar mass.

Ans : The molecular mass of a substance is the sum of the atomic masses of all atoms in a molecule, whereas the mass of 1 mole of any substance is called its molar mass.

46. Define the atomic mass unit. 

Ans : 

The mass of 1/12th part of C-12 is equivalent to one atomic mass unit. Previous, it was denoted by symbol ‘amu’ but nowadays it is denoted by symbol ‘u’.

47. Why is it not possible to see an atom with naked eyes?

Ans : Because an atom is too small, i.e., the atomic radii of 

an atom is of the order 10–19 m to 10–9 m.

48. What is meant by the term chemical formula?

Ans : The chemical formula of a compound is a symbolic 

representation of its composition and actual number of atoms in one molecule of a pure substance may be an atom or a compound.

49. If 9 g of water is decomposed, how many grams of hydrogen and oxygen are obtained?

Ans : If 9 g of water is decomposed, 1 g of hydrogen and 8 g of oxygen are always obtained.

50. Name the two laws of chemical combination.

Ans : 

Law of conservation of mass and law of constant proportions.

51. How was the relative atomic mass determined?

Ans : Relative atomic masses were determined by using the 

laws of chemical combinations and the compound formed.

52. What did Antoine L. Lavoisier observe regarding the formation of compound?

Ans : He noted that many compounds were composed of 

two or more elements. Each compound had the same elements in the same proportions.

53. Hydrogen and oxygen combine in the ratio of 1 : 8 by 

mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?

Ans : 1 g of hydrogen reacts with oxygen = 8 g

3 g of hydrogen reacts with oxygen = 8 × 3 g= 24 g

54. Write the chemical symbols of two elements which are formed from the first letter of the elements’ name.

Ans : N (Nitrogen), F (Fluorine), I (Iodine), O (Oxygen) (any two).

55. Why atoms form ions?

Ans : Atoms get stability by acquiring the stable electronic 

configuration of the nearest noble gas for which either they lose electrons or gain electrons and thus acquire noble gas configuration.

56. Give an example in each of the following cases :

(i) a divalent anion

(ii) a trivalent cation

(iii) a mono-valent anion.

Ans : 

(i) O2– (ii) Fe3+ (iii) I–

THREE MARKS QUESTIONS 

57. What is the ratio by mass of combining elements in H2O, CO2 and NH3 ?

Ans : H2O ratio by mass of combining elements= 2 : 16 = 1 : 8 (H : O)

CO2ratio by mass of combining elements

 = 12 : 32 = 3 : 8 (C : O)

NH3 ratio by mass of combining elements

 = 14 : 3 = 14 : 3 (N : H)

58. Calculate the ratio of the numbers of atoms for magnesium sulphide.

Ans : 

 Atomic mass of Mg= 3/24=1/8

 Atomic mass of S =4/32=1/8

 Ratio of atoms in MgS=1/8 : 1/8

or 1 : 1

59. Give the chemical name, chemical formulae for the 

following : Washing soda, blue vitriol, baking soda, green vitriol, oil of vitriol, soda ash, marble chips, lime water.

Ans : 

Compound 

Name

Chemical Name Chemical 

Formulae

Washing soda Sodium 

carbonate

Na2CO3.10H2O

Blue vitriol Copper sulphate CuSO4.5H2O

Baking soda Sodium bicarbonate

NaHCO3

Green vitriol Ferrods sulphate FeSO4.7H2O

Oil of vitriol Sulphuric acid H2SO4

Soda ash Sodium carbonate

Na2CO3

Lime water Calcium hydroxide

Ca(OH)2

Marble chips Calcium carbonate

CaCO3

60. Find the molecular mass of the following :H2, O2, CH4, CH3OH, CO2, HCl, Na2O, MgCl2, NaF, Na2CO3, NaNO3, H2SO4

Ans : 

H2 = 2 × 1 = 2u

O2 = 2 × 16 = 32u

CH4 = 1 × 12 + 4 × 1 = 16u

CH3OH = 1 × 12 + 3 × 1 + 1 × 16 

+ 1 × 1= 32u

CO2 = 1 × 12 + 2 × 16 = 44u

HCl = 1 × 1 + 1 × 35.5 = 36.5u

Na2O = 2 × 23 + 1 × 16 = 62u

MgCl2 = 1 × 24 + 2 × 35.5 = 95u

NaF = 1 × 23 + 1 × 19 = 42u

Na2CO3 = 2 × 23 + 1 × 12 + 3 × 16= 106u

NaNO3 = 1 × 23 + 1 × 14 + 3 × 16= 85u

H2SO4 = 1 × 2 + 1 × 32 + 16 × 4 = 98u

61. Calculate the following in 5.6 g of nitrogen :

(a) Number of moles of nitrogen

(b) Number of molecules of nitrogen

(c) Number of atoms of nitrogen

Ans : (a) Molar mass of nitrogen = 14 g

5.6 g of nitrogen = .

14

5 6 = = 0.4 mole

(b) 28 g of nitrogen = 6.022 × 1023 molecules

 5.6 g of nitrogen = 2.15 × 1022 molecules

(c) 14 g of nitrogen = 6.023 × 1023 atoms

 5.6 g of nitrogen = 4.30 × 1023 atoms

62. What is Avogadro number?

Ans : 

The number 6.022 × 1023 is referred to as Avogadro 

number and is denoted by symbol NA.

63. What are ionic compounds?

Ans : Ionic compounds are charged particles. Such 

compounds form by joining or losing or sharing the 

electron. For example : Sodium chloride is an ionic 

compound. Its constituent particles are positively 

charged sodium ion (Na+) and negatively charged 

chloride ion (Cl–).

64. What is the similarity between chlorine molecule, nitrogen molecule and hydrogen molecule?

Ans : 

Chlorine molecule, nitrogen molecule and hydrogen molecule are diatomic molecules. These are formed by the union of two atoms of the same element.

65. What is the difference between hydrogen chloride and 

nitrogen molecule formation?

Ans : Hydrogen chloride is molecular compound and formed by the union of different kinds of atoms while nitrogen is diatomic molecule and formed by union of two atoms of same kinds.

66. All elements have charged valency. Explain.

Ans : No, all elements do not form ions thus they, do not have a charge. For example : Carbon has a valency of 4 and nitrogen has a valency of 3. Non-metals are formed without a charged valency. Example : In carbon tetrachloride, carbon has valency of 4 and chlorine has a valency of 1

67. If the valency of an element X is 3, then write the chemical formula of its oxide.

Ans : 

68. Write the meaning of these formulae :

(i) 2O,(ii) O2,(iii) O3

Ans : 

(i) 2O = Two atoms of oxygen

(ii) O2= One molecule of oxygen

(iii) O3 = One molecule of ozone

69. Write the name of the compounds : NaBr, Al2O3, CaCO3.

Ans : NaBr = Sodium bromide

Al2O3= Aluminium oxide

CaCO3= Calcium carbonate

70. Out of these Na+, K+, Al3+ and O2–, which is iso-electronic? 

Ans : Iso-electronic means species having same number of 

electrons.Ions Atomic 

number of element

Atomic 

number of 

ions

Number of 

electron

Na+ 11 10 10

K+ 19 18 18

Al3+ 13 10 10

O2– 8 10 10

Na+, Al3+ and O2– have 10 electrons each Hence, they 

are iso-electronic.

71. Find the number of atoms in the 0.5 mole of C atom. 

Ans : 

0.5 mole of C atom :

Number of atoms in 1 mole of C atom = 6.022 × 1023 atoms

Number of atoms in 0.5 mole of C atom = 6.022 × 1023 × 0.5

 = 3.011 × 1023 atoms

72. Write the postulate given by the Indian philosopher 

Maharishi Kanad.

Ans : Indian philosopher Maharishi Kanad postulated if we divide matter we will get smaller and smaller particles. 

He said that a time will come when we come across smallest particles beyond which further division will not be possible.

73. What is the significance of symbol of an element? Explain with an example.

Ans : 

(i) The symbol of an element represents the name of 

the element.

(ii) It represents one atom of the element.

(iii) It represents a definite mass of the element.

For example :

(i) The symbol ‘H’ represents the element hydrogen.

(ii) The symbol ‘H’ represents one atom of the element hydrogen.

(iii) The symbol ‘H’ represents 1u.

74. How can Dalton’s atomic theory explain the Law of Constant Proportions?

Ans : According to Dalton’s atomic theory, atoms of the 

same elements are same. Also atoms combine in whole number. This means that the atoms can combine with each other in a simple fixed ratio to form molecules.

75. Give one example each of (i) Monovalent cation, (ii) Bivalent cation, (iii) Monovalent anion, (iv) Bivalent anion.

Ans : 

(i) K+ or Na+

(ii) Mg+2 or Ca+2

(iii) F–or Cl–

(iv) O2– or S2–

76. What is meant by a chemical formula? Give examples. 

Ans : A chemical formula of a compound shows its constituent elements and the number of atoms of each combining element, e.g., chemical formula of ammonia is NH3, water is H2O and carbon dioxide is CO2.

77. What is meant by a molecule? Give examples.

Ans : A molecule is the smallest particle of an element or a compound capable of independent existence under 

ordinary conditions. It shows all the properties of the substance, e.g., molecule of oxygen is O2, ozone is O3, phosphorus is P4, sulphur is S8, etc.

78. Define one mole, give its relationship with Avogadro 

constant.

Ans : 

One mole of any species (atoms, molecules, ions or particles) is that quantity in number having a mass equal to its atomic or molecular mass in grams. The 

number of particles (atoms, molecules or ions) present 

in 1 mole of any substance is fixed, with a value of 6.022 × 1023. This number is called Avogadro constant or Avogadro number.

79. What is the mass of :

(a) 0.2 mole of oxygen atoms?

(b) 0.5 mole of water molecules?

Ans : 

(a) 1 mole of oxygen atoms = 1 × 16 = 16 g

0.2 mole of oxygen atoms = 16 g × 0.2 = 3.2 g

(b) 1 mole of water (H2O) molecules

 = 2 × 1 g + 1 × 16 g = 18 g

 0.5 mole of water (H2O) molecules

 = 18 g × 0.5 = 9.0 g

80. State two examples in each case and write their chemical formulae :

(a) Molecules having same kind of atoms only.

(b) Molecules having two different kinds of atoms.

(c) Molecules having three different kinds of atoms.

Ans : (a) F2, Cl2, P4, S8

(b) Ammonia (NH3), Sulphur dioxide (SO2), Carbon disulphide (CS2).

(c) Calcium sulphate (CaSO4), Sodium nitrate (NaNO3).

81. Write the names of the following compounds :

(a) NiS

(b) Mg(NO3)2

(c) Na2SO4

(d) Al(NO3)3

(e) K3PO4

(f) Ca3N2

Ans : 

(a) Nickel sulphide,

(b) Magnesium nitrate,

(c) Sodium sulphate,

(d) Aluminium nitrate,

(e) Potassium phosphate,

(f) Calcium nitride.

82. The atomic mass of an element is in fraction.” What does it mean?

Ans : If the atomic mass of an element is in fraction, this means that it exists in the form of isotopes. The atomic mass of such element is the average of atomic masses of its isotopes and is generally in fraction.

83. Why the number of atoms in one mole of hydrogen gas is double the number of atoms in one mole of helium gas? Explain.

Ans : The number of atoms in one mole of hydrogen gas is double the number of atoms in one mole of helium gas because hydrogen molecule is diatomic, i.e., a molecule of hydrogen consists of two atoms of hydrogen, whereas 

helium is monatomic.

84. Write three points of difference between an atom and a molecule.

or

What is the difference between an atom and a molecule?

Ans : S. No.

Atom Molecule

1. Atom is the smallest 

particle of an element 

that takes part in a 

chemical reaction.

Smallest particle 

of an element or 

compound that can 

exist independently.

2. An atom is usually 

not stable itself.

A molecule is usually 

stable itself.

3. When similar atoms 

combine together in 

varying numbers, 

molecules of different 

properties can be 

formed. E.g. O2

, O3

.

When same molecules 

combine together 

in any numbers, 

a simple similar 

product is formed.

85. Define formula unit mass. Calculate formula unit mass of NaCl (atomic mass of Na = 23u, Cl = 35.5u).

Ans : The formula unit mass is same as molecular mass which is equal to the sum of masses of atoms present in a formula unit. Formula unit mass of NaCl = (23 

+ 35.5) = 58.5u.

86. Calculate the mass of the following :

(i) 2 moles of carbon dioxide.

(ii) 6.022 × 1023 molecules of carbon dioxide.

Ans : 

(i) Molar mass of CO2

 = 12 + 2 × 16 g = 44 g

 1 mole of carbon dioxide = 44 g

 2 mole of carbon dioxide = 44 g × 2 = 88 g

(ii) Molar mass of CO2

 = 44 g

 = 6.022×1023 molecules of carbon dioxide

87. Write the symbols of the following elements : Aluminium, Argon, Barium, Bromine, Beryllium, Calcium, Cobalt, Chlorine, Chromium, Helium, Lithium, Magnesium, Manganese, Neon, Nickel, Silicon, and Platinum.

Ans : Element Symbol Element SymbolAluminium Al Argon Ar

Barium Ba Bromine Br

Beryllium Be Calcium Ca

Cobalt Co Chlorine Cl

Chromium Cr Helium He

Lithium Li Magnesium Mg

Manganese Mn Neon Ne

Nickel Ni Silicon Si

Platinum Pt

88. Name the scientists whose experimentation established laws of chemical combination. Name the laws also.

Ans : Antoine Laurent Lavoisier and Joseph L. Proust experimented and established two laws of chemical combination.

These laws are :

(i) Law of conservation of mass,

(ii) Law of constant proportions.

89. Give two drawbacks of Dalton’s atomic theory.

Ans : Drawbacks of Dalton’s atomic theory :

(i) According to modern theory, atom is not the ultimate indivisible particle of matter. Atoms are divisible, i.e., they are themselves made-up of particles (protons, electrons, neutrons, etc.).

(ii) The assumption that the atoms of the same element have same mass does not hold good, in case of isotopes of an element.

90. Calculate molar mass of sulphuric acid.

Ans : Formula of sulphuric acid = H2SO4

Molar mass of H2SO4

 = 2 × mass of H

 + 1 × mass of S + 4 × mass of O

 = 2 × 1 + 1 × 32 + 4 × 16

 = 2 + 32 + 64 = 98 gmol–1

91. Give the electronic configuration of : Al atom and its 

ion.

Ans : 

Al atom and its ion

Al atom K L M

283

Al3+ K L

2 8

92. Sunita calculated molecular mass of S8molecule and reported it as 64u from the given atomic mass of 32u. But teacher considered her answer as wrong. What is the correct molecular mass of S8? Calculate the 

number of moles of S8 in 25.6 g of the sample.

Ans : Molecular mass of S8

 = 32 × 8 = 256u

Moles of S8in 25.6 g sample .

256

25 6 = = 0.1 mole

FIVE MARKS QUESTIONS

93. Glucose has the molecular formula C6H12O6. Calculate :

(a) Its molecular mass.

(b) The number of atoms in one molecule of glucose.

(c) The number of gram molecule in 18 g of glucose.

Ans : 

(a) Molecular mass of C6H12O6

 = (6 × 12u) + (12 × 1u) + (6 × 16u)

 = 72u + 12u + 96u = 180u

(b) The number of atoms in one molecule of C6H12O6

 = 6 atoms of C + 12 atoms of H

 + 6 atoms of O

 = 6 + 12 + 6 = 24 atoms

(c) Number of gram molecules

 Molecular mass of glucose(g)

Mass of glucose(g) = . 180

18 = = 0 1

94. What is the mass of (i) 2.5 moles of CO2 and (ii) 1 mole of water?

Ans : 

(i) 1 mole of CO2

 = Molecular mass expressed in 

grams

 = 1 × 44 g

 2.5 moles of CO2

 = 2.5 × 44 = 110 g

(ii) Mass of the substance = Moles of substance

 × Molecular mass in grams

 Mass of water = 1 × 18 g = 18 g

95. Calculate the number of H2

O molecules in one drop of water having a mass of 0.05 g.

Ans : 

Number of moles of H2O in 0.05 g of water .

g

g

18

0 05 = (molar mass of water = 18)

 . .

g

g

18

0 05 6 022 1023

= # #

 = 1.673 × 1023 molecules

96. What is the mass percentage of different elements in calcium carbonate? (Atomic mass : Ca = 40, O = 16)

Ans : 

Molecular mass of CaCO3

 = At. Mass of Ca + At. 

Mass of C + 3 × At. Mass of O

 = 40 + 12 + 3 × 16 = 100

Mass percentage of Ca

 Mass of molecule

Mass of element = # 100

 % 100

40 = = # 100 40

 C % 100

12 = = # 100 12

 O %

97. What is the use of mole concept? 

Ans : 

Applications of mole concept :

(i) We can calculate the number of basic particles from the number of moles as the number of moles of a substance is directly proportional to the number of elementary particles.

(ii) One mole of gas occupies 22.4 litres at 273K.

(iii) One mole of any gas occupies the same volume at same pressure and temperature.

(iv) One mole is equal to 6.022 × 1023 atoms. So, we can calculate the absolute masses of atoms and molecules.

98. Define the term gram atom. How is it related to mole and Avogadro number?

Ans : The atomic mass of an element expressed in grams 

is called gram atomic mass. One gram atom of any element contains 6.022 × 1023 atoms of the element. It lis equal to one mole of atoms.

One gram atomic mass = 6.022 × 1023 atoms = 1 mole

99. Give symbol and valency of : Potassium, Barium, Aluminium, Calcium, Cobalt, Fluorine, Lead, Zinc, Iodine, Sulphide.

Ans : Name of Ion Symbol Valency

Potassium K+ +1

Barium Ba2+ +2

Aluminium Al3+ +3

Calcium Ca2+ +2

Cobalt Co2+ +2

Fluorine F– –1

Lead Pb+ +1

Zinc Zn2+ +2

Iodine I– –1

Sulphide S2– –2

100. Ca2P2lO7is the formula of calcium pyrophosphate. Write the formula for ferric pyrophosphate.

Ans : 

Valency of calcium is +2. Ca2P2O7 has two calcium atoms. So, calcium have total of +4 charges. Thus, pyrophosphate has a valency of –4. Since ferric ion has a valency of +3, the formula of ferric pyrophosphate 

is Fe4(P2O7)3.

101. The mass of any single atom X is 3.05 × 10–22 g. What is its atomic weight? Name the possible element.

Ans : 

1 mole = atomic mass

 = 6.022 × 1023 atoms

Now, mass of one atom of X = 3.05 × 10–22 g

Mass of 6.022 × 1023 atoms of X

 = 3.05 × 10–22 × 6.022 × 1023 g

 = 183.7 g

This element could be tungsten.

102. Write formula for the following :

(a) Zinc sulphate,

(b) Methane,

(c) Ammonium carbonate.

Ans : (a) Zinc sulphate

Thus, Zn2(SO4l)2and finally = ZnSO4

(b) Methane Thus, finally = CH4

(c) Ammonium carbonate

Thus, finally = (NH4)2CO3

103. 50 g of 10% lead nitrate is mixed with 50 g of 10% sodium chloride in a closed vessel. It was found after reaction that 6.83 g of lead chloride was precipitated. Besides, the reaction mixture contained 90 g water and sodium nitrate. Calculate the amount of sodium nitrate formed.

Ans : 

 50 g of 10% lead nitrate = 5 g lead nitrate + 45 g water

 50 g of 10% sodium chloride = 5 g sodium chloride

 + 45 g water

 Total content before reaction = 5 + 5 + 90 = 100

 Total content after reaction = 90 g

 Amount of precipitate = 6.83 g

According to law of conservation,

Total mass of reaction mixture = 100 g

Amount of sodium nitrate = 100 – 90 – 6.83 = 3.17 

g

104. Explain the law of multiple proportions.

Ans : 

According to law of multiple proportions, when two elements combine to make one or more compounds then the ratio of weights of these element remain in lfixed ratio to one another. For example : Hydrogen and oxygen combine to form water (H2O) and hydrogenperoxide (H2O2) under different condition. 2 grams of hydrogen combines with 16 grams of oxygen in case of water while 2 grams of hydrogen combines with 32 grams of oxygen to form hydrogen peroxide. Now, lthe weights of oxygen combine with a fixed weight of hydrogen in water and hydrogen peroxide respectively are 16 and 32 which are in simple ratio of 16: 32 or 1 : 2.

105. Explain the form of atoms in a solid.

Ans : A solid element is a cluster of atoms. The property of solid does not depend on a single atom but on cluster of atoms. 

For example : Diamond and graphite though both are composed of carbon atoms but due to different arrangements of carbon atoms in these. 

They are different in physical and chemical properties.

106. What are molecules? Give brief explanation of the 

arrangement of the constituent atoms in the molecules.

Ans : 

A molecule is the smallest particle of an element or 

compound which is stable in normal conditions. And 

it can freely show all the properties of that element 

or compound. It may be made up of one, two or more 

atoms. Molecule with one atom called monoatomic. 

E.g. helium, neon, etc.

Molecule with two atoms called diatomic. E.g. 

Cl2, O2. Similarly, there are molecules containing three 

atoms (CO2), four atoms (P4) and so on.

107. The mass of one molecule of a substance is 4.65 × 1023

grams. What is its molecular mass?

Ans : 

Mass of 1 molecule of a substance = 4.65 × 1023 grams

Mass of 6.023 × 10–23 molecules of a substance

 = 4.65 × 1023 × 6.023 × 10–23

 = 28 g

Molecular mass of the substance = 28 g

108. Chlorine occurs in nature in two isotopic forms with 

masses 35u and 37u in the ratio of 3 : 1. What should 

be the mass of chlorine atom?

Ans : 

35 100

75 37 100

25 # # + 4

105

4

37 = +

 . u 4

142 = = 35 5

109. An element 12X24 loses two electron to form a cation which combines with the anion of element 17Y35 formed by gaining an electron.

(a) Write the electronic configuration of element X.

(b) Write the electronic configuration of the anion of element Y.

(c) Write the formula for the compound formed by combination of X and Y.

Ans : 

(a) X = 2, 8, 2

(b) Y–

 = 2, 8, 8

(c) XY2

110. Calculate the formula unit masses of ZnO, Na2O, K2CO3

 given atomic masses of Zn = 65u, Na = 23u, K 

= 39u, C = 12u, and 0 = 16u.

Ans : 

 Formula unit mass of ZnO = 1 × 65u + 1 × 16u= 81u

 Formula unit mass of Na2O = 2 × 23u + 1 × 16u= 62uFormula unit mass of K2CO3

 = 2 × 39u + 1 × 12u + 3 × 16u

 = 138u

111. If one mole of carbon atoms weighs 12 grams, what is the mass (in gram) of 1 atom of carbon?

Ans : 

1 mole of carbon weighs = 12 g

1 atom of carbon weighs .6 022 10

12 23 # =

 = 1.99 × 10–23 g

112. What is the mass of :

(a) 1 mole of nitrogen atoms?

(b) 4 moles of aluminium atoms (atomic mass of aluminium = 27)?

(c) 10 moles of sodium sulphite (Na2SO3)?

Ans : 

(a) 1 mole of nitrogen atoms

 = 1 × gram atomic mass of nitrogen atom

 = 1 × 14 g = 14 g

(b) 4 moles of aluminium atoms

 = 4 × gram atomic mass of aluminium atoms

 = 4 × 27 g = 108 g

(c) 10 moles of sodium sulphite (Na2SO3)

 = 10 (2 × gram atomic mass of Na

 + 1 × gram atomic mass of sulphur

 + 3 × gram atomic mass of oxygen)

 = 10 (2 × 23 g + 1 × 32 g + 3 × 16g)

 = 10 (46 g + 32 g + 48 g)

 = 10 × 126 g = 1260 g

113. Give the postulates of Dalton’s atomic theory.

Ans : 

Every element is composed of extremely small particles 

called atoms. Atoms of a given element are identical, 

both in mass and properties. Different chemical 

elements have different kinds of atoms; in particular, their atoms have different masses.Atoms cannot be created, destroyed or transformed into atoms of other elements. Compounds are formed when atoms of different elements combine with each other in small whole number ratios. The relative number and kinds of atoms in a given compound are constant.

114. (a) Give one point of difference between an atom and 

an ion.

(b) Give one example each of a polyatomic cation and an anion.

(c) Identify the correct chemical name of FeSO3: Ferrous sulphate, Ferrous sulphide, Ferrous sulphite.

(d) Write the chemical formula for the chloride of magnesium.

Ans : 

(a) An atom is electrically neutral while an ion is electrically charged particle.

(b) (i) Polyatomic cation : (NH4)+

 (ii) Polyatomic anion : (SO4)2–

(c) Ferrous sulphite

(d) MgCl2(Magnesium chloride)

115. Q. 115. When 3.0 g of magnesium is burnt in 2.00 g of 

oxygen, 5.00 g of magnesium oxide is produced. What mass of magnesium oxide will be formed when 3.00 g magnesium is burnt in 5.00 g of oxygen? Which law of lchemical combination will govern your answer? State the law.

Ans : 

When 3.0 g of magnesium is burnt in 2.00 g of oxygen, 

5.00 g of magnesium oxide is produced. It means 

magnesium and oxygen are combined in the ratio of 

3 : 2 to form magnesium oxide.

Thus, when 3.00 g of magnesium is burnt in 5.00 

g of oxygen, 5.00 g of magnesium oxide will be formed 

and the remaining oxygen will be left unused. It is 

governed by law of definite proportions.

It states that in a chemical substance, the elements 

are always present in definite proportions by mass.

116. A sample of ethane (C2

H6) gas has the same mass as 

1.5 × 10²⁰ molecules of methane (CH4). How many (C2H6) molecules does the sample of gas contain?

Ans : 

 Mass of 1 molecule of CH4 N g 16

A

=

Mass of 1.5 × 1020 molecule of CH4

 . N 1 5 10 g 16

A

20 = # #

 Mass of 1 molecule of C2

H6 N g 30

A

=

Mass of 1.5 × 1020 molecule of C2

H4

 . N 1 5 10 g 16

A

20 = # #

Mass of 1.5 × 1020 molecule of ethane

 . N

N 1 5 10 16

A 30 = # ## 20 A

 = 0.8 × 1020

117. (a) Calculate the number of molecules of SO2

 present 

in 44 g of it.

(b) If one mole of oxygen atoms weighs 16 grams, find 

the mass of one atom of oxygen in grams.

Ans : 

(a) Molecular mass of SO2

 = Atomic mass of S

 + 2 × Atomic mass of O

 = 32 + 2 × 16 = 64u

 Molar mass = 64 g

Number of molecules, N

 = Given mass '

Molar mass

Avogadro s number #

 . 64

44 6 022 1023 = # #

 = 4.14 × 1023 molecules

(b) One mole of oxygen contains 6.022 × 1023 atoms 

of oxygen

 Mass of one atom of oxygen 6. g 022 10

16 23 # =

 = 2.66 × 10–23 g

118. Sodium is represented as 23Na11.

(a) What is its atomic mass?

(b) Write its gram atomic mass.

(c) How many atoms of Na will be there in 11.5 g of 

the sample?

Ans : 

(a) Atomic mass = 23u

(b) Gram atomic mass = 23 g

(c) Given mass = 11.5 g 

 Molar mass = 23 g

 Number of atoms (N)

 = (Given mass × Avogadro s number) / Molar mass

=(11.5 × 6.022 × 10²³)/23

 = 3.011×10²³ atoms