Class 9 Science Chapter 3 Atoms and Molecules
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Atoms and Molecules
Answers to Questions Within the Lesson(1)
1. Q. In a reaction, 5.3 g of sodium carbonate reacted with 6 g of ethanoic acid. The products formed were 2.2 g of carbon dioxide, 0.9 g of water and 8.2 g of sodium ethanoate. Show that the results are in accordance with the law of conservation of mass.
Ans: Total mass of reactants = 5.3 g + 6 g = 11.3 g
Total mass of products = 2.2 g + 0.9 g + 8.2 g = 11.3 g
Since the total mass of reactants equals the total mass of products, the reaction follows the law of conservation of mass.
2. Q. Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen will be required to react completely with 3 gof hydrogen?
Ans: Given ratio of hydrogen : oxygen = 1 : 8
So, for 1 g hydrogen, oxygen required = 8 g
For 3 g hydrogen, oxygen required = 3 × 8 = 24 g
3. Q. Which postulate of Dalton’s atomic theory is the result of the lawof conservation of mass?
Ans: Atoms are indivisible particles that are neither created nor destroyed in a chemical reaction.
4. Q. Which postulate of Dalton’s atomic theory explains the law of constant proportions?
Ans: In a chemical compound, elements are always present in fixed proportions by mass.
Answers to Questions Within the Lesson(2)
1. Q. Define atomic mass unit.
Ans: One atomic mass unit is defined as one-twelfth (1/12) of the mass of one carbon-12 atom.
2. Q. Why is it not possible to see an atom with the naked eye?
Ans: Atoms are extremely small in size, with radii of the order of 10?¹°metres. Therefore, they cannot be seen with the naked eye.
Answers to Questions Within the Lesson(3)
1. Q. Write the chemical formulae.
(i) Sodium oxide
(ii) Aluminium chloride
(iii) Sodium sulphide
(iv) Magnesium hydroxide
Ans: (i) Na2O
(ii) AlCl3
(iii) Na2S
(iv) Mg(OH)2
2. Q. Write the names of the compounds represented by the following formulae.
(i) Al2 (SO4)3
(ii) CaCl2
(iii) K2SO4
(iv) KNO3
(v) CaCO3
Ans:
(i) Aluminium sulphate
(ii) Calcium chloride
(iii) Potassium sulphate
(iv) Potassium nitrate
(v) Calcium carbonate
3. Q. What is meant by the term chemical formula?
Ans: A chemical formula represents the composition of a substance in terms of the symbols of elements and the number of atoms of each element present in one molecule or formula unit.
4 Q. How many atoms are present in (i) H2S molecule and (ii) PO4³-? ion?
Ans:
(i) H2S has 3 atoms (2 hydrogen + 1 sulphur).
(ii) PO4 ³-? has 5 atoms (1 phosphorus + 4 oxygen).
Answers to Questions Within the Lesson(4)
1. Q. Calculate the molecular masses of H2
, O2
, Cl2
, CO2
, CH4, C2H6, C2H4
, NH3
and CH3OH.
Ans:
H2
= 2 × 1 = 2 u
O2
= 2 × 16 = 32 u
Cl2
= 2 × 35.5 = 71 u
CO2
= 12 + (2 × 16) = 44 u
CH4
= 12 + (4 × 1) = 16 u
C2H6
= (2 × 12) + (6 × 1) = 30 u
C2H4
= (2 × 12) + (4 × 1) = 28 u
NH3
= 14 + (3 × 1) = 17 u
CH3OH = 12 + (4 × 1) + 16 = 32 u
Answers to Questions Within the Lesson(5)
1. Q. If the mass of one mole of carbon atoms is 12 g, what is the massof one carbon atom (in grams)?
Ans:
1 mole of carbon atoms = 6.022 × 10²³ atoms
Mass of 1 atom = 12 g / (6.022 × 10²³)
= 1.99 × 10-
²³ g (approximately)
2. Q. Which contains more number of atoms: 100 g of sodium or 100 gof iron? (Given: Atomic mass of Na = 23 u, Fe = 56 u)
Ans:
Number of moles of Na = 100 / 23 ˜ 4.35 mol
Number of moles of Fe = 100 / 56 ˜ 1.79 mol
Since sodium has more moles, it contains more atoms.
Exercise: Questions and Answers
1. Q. A sample of a compound formed by oxygen and boron weighing0.24 g was analysed and found to contain 0.096 g of boron and 0.144 g ofoxygen. Calculate the percentage composition of the compound by mass.
Ans: Percentage of boron = (0.096 / 0.24) × 100 = 40%Percentage of oxygen = (0.144 / 0.24) × 100 = 60%2. Q. 3.0 g of carbon burns in 8.00 g of oxygen to produce 11.00 g of carbon dioxide. What will be the mass of carbon dioxide formed when3.00 g of carbon is burnt in 50.00 g of oxygen? Which law of chemical
combination will be applicable?
Ans: 3.0 g carbon reacts with 8.00 g oxygen to form 11.00 g carbon dioxide.
Again, 3.00 g carbon with 50.00 g oxygen will produce only 11.00 g carbon dioxide.
The remaining oxygen = 50 – 8 = 42 g (unreacted).
This follows the law of constant proportions.
3. Q. What are polyatomic ions? Give examples.
Ans: Polyatomic ions are groups of two or more atoms that carry a net
charge and act as a single unit in chemical reactions. For example: SO4²?,
NO3
–
, NH4+
.
4. Q. Write the chemical formulae of the following.
(a) Magnesium chloride
(b) Calcium oxide
(c) Copper nitrate
(d) Aluminium chloride
(e) Calcium carbonate
Ans: (a) MgCl2
, (b) CaO, (c) Cu(NO3
)2
, (d) AlCl3
, (e) CaCO35. Q. Name the elements present in the following compounds.
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate
Ans:
(a) Quick lime: Calcium and oxygen.
(b) Hydrogen bromide: Hydrogen and bromine.
(c) Baking powder: Sodium, hydrogen, carbon and oxygen.
(d) Potassium sulphate: Potassium, sulphur and oxygen.
6. Q. Calculate the molar masses of the following substances.
(a) Ethane (C2H6
)
(b) Sulphur molecule (S8
)
(c) Phosphorus molecule (P4
), (Atomic mass of P = 31)
(d) Hydrochloric acid (HCl)
(e) Nitric acid (HNO3
)
Ans:
(a) C2H6
= (2 × 12) + (6 × 1) = 30 g/mol
(b) S8
= 8 × 32 = 256 g/mol
(c) P4
= 4 × 31 = 124 g/mol
(d) HCl = 1 + 35.5 = 36.5 g/mol
(e) HNO3
= 1 + 14 + (3 × 16) = 63 g/mol
7. Q. Find the mass of—
(a) 1 mole of nitrogen atoms
(b) 4 moles of aluminium atoms (Atomic mass of Al = 27)
(c) 10 moles of sodium sulphite (Na2SO3
)
Ans:
(a) 1 mole of nitrogen atoms = 14 g
(b) 4 moles of aluminium = 4 × 27 = 108 g
(c) Molar mass of Na2SO3
= (2×23) + 32 + (3×16) = 126 g/mol
So, 10 moles = 10 × 126 = 1260 g
8. Q. Convert into moles—
(a) 12 g of oxygen gas (O2
)
(b) 20 g of water (H2O)
(c) 22 g of carbon dioxide (CO2
)
Ans:
(a) Moles = 12 / 32 = 0.375 mol
(b) Moles = 20 / 18 ˜ 1.11 mol
(c) Moles = 22 / 44 = 0.5 mol
9. Q. Find the mass of—
(a) 0.2 mole of oxygen atoms
(b) 0.5 mole of water molecules
Ans:
(a) Mass = 0.2 × 16 = 3.2 g
(b) Mass = 0.5 × 18 = 9 g
10. Q. Calculate the number of sulphur molecules (S8
) present in 16 gof solid sulphur.
Ans:
Molar mass of S8
= 256 g/mol
Moles = 16 / 256 = 1/16 mol
Number of molecules = (1/16) × 6.022 × 10²³ ˜ 3.76 × 10²² molecules11. Q. Calculate the number of aluminium ions present in 0.051 g ofaluminium oxide. (Atomic mass of Al = 27)
Ans:
Molar mass of Al2O3 = (2×27) + (3×16) = 102 g/mol
Moles of Al2O3 = 0.051 / 102 = 0.0005 mol
Number of formula units = 0.0005 × 6.022 × 10²³
= 3.011 × 10²°
Each unit has 2 Al ions
Total Al ions = 2 × 3.011 × 10²° = 6.022 × 10²° ions
Additional Questions & Answers
1. Q. Who proposed the laws of chemical combination? Name the lawsand define them.
Ans: The laws of chemical combination were proposed by Joseph L. Proust.
(i) Law of conservation of mass: Mass can neither be created nor destroyedin a chemical reaction.
(ii) Law of constant proportions: In a chemical compound, elements arealways present in fixed proportions by mass.
2. Q. Write the postulates of Dalton’s atomic theory.
Ans:
(i) All matter is made up of tiny particles called atoms.
(ii) Atoms are indivisible and cannot be created or destroyed in a chemical
reaction.
(iii) Atoms of the same element have identical mass and chemical properties.
(iv) Atoms of different elements have different masses and properties.
(v) Atoms combine in simple whole-number ratios to form compounds.
(vi) In a given compound, the relative number and types of atoms remain constant.
3. Q. What is an atom?
Ans: The smallest particle of an element that can take part in a chemical reaction is called an atom.
4. Q. What is the unit used to measure atomic size?
Ans: Nanometre.
5. Q. What is atomic mass?
Ans: The mass of an atom of an element relative to 1/12th the mass of acarbon-12 atom is called its atomic mass.
6. Q. What is a molecule? Give examples.
Ans: The smallest particle of an element or compound that can exist independently and shows all its properties is called a molecule. For example: H2, N2
, H2O.
7. Q. What is an ion? Give examples.
Ans: Charged particles formed from atoms or groups of atoms are calledions. For example: Na+
, Cl-
, K+
, H+
.
8. Q. What is valency?
Ans: The combining capacity of an element is called its valency.
9. Q. What is molecular mass?
Ans: The mass of a molecule relative to 1/12th the mass of a carbon-12atom is called molecular mass.
10. Q. What is Avogadro’s number?
Ans: Avogadro’s number is 6.022 × 10²³.
11. Q. Define mole.
Ans: A mole is the amount of substance that contains as many particles as there are atoms in exactly 12 g of carbon-12.
12. Q. What is molar mass?
Ans: The mass of one mole of a substance is called its molar mass.
Multiple Choice Questions (MCQ)
1. What is an atom?
(a) Smallest particle of a compound
(b) Smallest particle of an element that takes part in a chemical reaction
(c) A charged particle
(d) A mixture
Ans: (b)
2. Who proposed the atomic theory?
(a) Rutherford
(b) Dalton
(c) Bohr
(d) Thomson
Ans: (b)
3. Which law states that mass can neither be created nor destroyed?
(a) Law of constant proportions
(b) Law of conservation of mass
(c) Law of multiple proportions
(d) Law of definite composition
Ans: (b)
4. Which law states that elements combine in fixed proportions by mass?
(a) Law of conservation of mass
(b) Law of constant proportions
(c) Law of gaseous volumes
(d) Law of energy conservation
Ans: (b)
5. What is the atomic mass unit (u)?
(a) Mass of hydrogen atom
(b) 1/12th mass of carbon-12 atom
(c) Mass of oxygen atom
(d) Mass of neutron
Ans: (b)
6. Why can atoms not be seen with the naked eye?
(a) They are colourless
(b) They are very small
(c) They move fast
(d) They are transparent
Ans: (b)
7. Which of the following is a molecule?
(a) Na
(b) O₂
(c) Fe
(d) Cu
Ans: (b)
8. Which of the following is a compound?
(a) Hydrogen
(b) Oxygen
(c) Water
(d) Nitrogen
Ans: (c)
9. What is the formula of sodium chloride?
(a) Na₂Cl
(b) NaCl
(c) NaCl₂
(d) Na₂Cl₂
Ans: (b)
10. What is the formula of water?
(a) H₂O
(b) HO₂
(c) H₂O₂
(d) H₃O
Ans: (a)
11. What is the valency of oxygen?
(a) 1
(b) 2
(c) 3
(d) 4
Ans: (b)
12. What is the valency of hydrogen?
(a) 1
(b) 2
(c) 3
(d) 4
Ans: (a)
13. Which of the following is a polyatomic ion?
(a) Na+
(b) Cl-
(c) SO₄²+
(d) H⁺
Ans: (c)
14. What is the formula of calcium carbonate?
(a) CaCO²
(b) CaCO₃
(c) Ca₂CO₃
(d) Ca(CO₃)₂
Ans: (b)
15. What is the formula of aluminium oxide?
(a) AlO
(b) Al₂O₃
(c) AlO₂
(d) Al₂O₂
Ans: (b)
16. What is the molecular mass of H2?
(a) 1 u
(b) 2 u
(c) 4 u
(d) 16 u
Ans: (b)
17. What is the molecular mass of O₂?
(a) 16 u
(b) 32 u
(c) 48 u
(d) 8 u
Ans: (b)
18. What is the molecular mass of CO₂?
(a) 28 u
(b) 32 u
(c) 44 u
(d) 40 u
Ans: (c)
19. What is Avogadro’s number?
(a) 6.022 × 10²²
(b) 6.022 × 10²³
(c) 6.022 × 10²¹
(d) 6.022 × 10²4
Ans: (b)
20. One mole of a substance contains—
(a) 6.022 × 10²² particles
(b) 6.022 × 10²³ particles
(c) 6.022 × 10²⁴particles
(d) 6.022 × 10²¹ particles
Ans: (b)
21. What is the molar mass of water?
(a) 16 g/mol
(b) 18 g/mol
(c) 20 g/mol
(d) 22 g/mol
Ans: (b)
22. What is the molar mass of CO₂?
(a) 44 g/mol
(b) 32 g/mol
(c) 28 g/mol
(d) 18 g/mol
Ans: (a)
23. Which of the following is an ion?
(a) H2O
(b) Na+
(c) CO2
(d) O2
Ans: (b)
24. What is valency?
(a) Mass of atom
(b) Combining capacity of atom
(c) Size of atom
(d) Charge of atom
Ans: (b)
25. What is the formula of ammonium ion?
(a) NH₃
(b) NH₄+
(c) NH₂
(d) NH₄
Ans: (b)
26. What is the formula of nitrate ion?
(a) NO₂-
(b) NO₃-
(c) N₂O
(d) NO₄
Ans: (b)
27. What is the molar mass of NaCl?
(a) 23 g/mol
(b) 35.5 g/mol
(c) 58.5 g/mol
(d) 60 g/mol
Ans: (c)
28. What is the mass of 1 mole of carbon atoms?
(a) 10 g
(b) 12 g
(c) 14 g
(d) 16 g
Ans: (b)
29. Which of the following is an element?
(a) Water
(b) Carbon dioxide
(c) Oxygen
(d) Sodium chloride
Ans: (c)
30. Which of the following is a compound?
(a) Nitrogen
(b) Hydrogen
(c) Methane
(d) Oxygen
Ans: (c)
31. What is the formula of methane?
(a) CH₃
(b) CH₄
(c) C₂H₄
(d) C₂H₆
Ans: (b)
32. What is the formula of ammonia?
(a) NH₂
(b) NH₃
(c) NH₄
(d) N₂H₄
Ans: (b)
33. What is the molar mass of NH₃?
(a) 14 g/mol
(b) 17 g/mol
(c) 16 g/mol
(d) 18 g/mol
Ans: (b)
34. What is the molar mass of CH₄?
(a) 12 g/mol
(b) 14 g/mol
(c) 16 g/mol
(d) 18 g/mol
Ans: (c)
35. What is the molar mass of CaCO₃?
(a) 100 g/mol
(b) 98 g/mol
(c) 102 g/mol
(d) 96 g/mol
Ans: (a)
36. Which of the following is a diatomic molecule?
(a) CO₂
(b) O₂
(c) CH₄
(d) NH₃
Ans: (b)
37. What is the atomic mass of oxygen?
(a) 8 u
(b) 16 u
(c) 32 u
(d) 14 u
Ans: (b)
38. What is the atomic mass of hydrogen?
(a) 1 u
(b) 2 u
(c) 3 u
(d) 4 u
Ans: (a)
39. Which of the following has maximum number of molecules?
(a) 1 mole of any substance
(b) 2 moles of any substance
(c) 0.5 mole
(d) 1 gram
Ans: (b)
40. What is the mass of 1 mole of O₂gas?
(a) 16 g
(b) 32 g
(c) 48 g
(d) 64 g
Ans: (b)
