CBSE Class 9 Science Atoms and Molecules Sure Shot Questions A

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1. Who established the two important laws of chemical combinations?
2. What are the laws of chemical combinations?
3. What is the law of conservation of mass?
4. Give an example to show Law of conservation of mass applies to physical change also.
5. Explain with example that law of conservation of mass is valid for chemical reactions.
6. The 2.8 g of nitrogen gas was allowed to react with 0.6 g of hydrogen gas to produce 3.4 g of ammonia. Show that these observations are in agreement with the law of Conservation of mass.
7. If 12 g of carbon is burnt in the presence of 32 g of oxygen, how much carbon dioxide will be formed?
8. Who proposed Law of Definite Proportions (or Law of Constant Composition)?
9. State Law of constant proportions. Explain with an example.
10. Show that water illustrates the law of constant proportions.
11. 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?
12. A 0.24 g sample of compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage composition of the compound by weight.
13. When 3.0 g of carbon is burnt in 8.00 g oxygen, 11.00 g of carbon dioxide is produced. What mass of carbon dioxide will be formed when 3.00 g of carbon is burnt in 50.00 g of
oxygen? Which law of chemical combination will govern your answer?
14. Magnesium and oxygen combine in the ratio of 3 : 2 by mass to form magnesium oxide. How much oxygen is required to react completely with 12 g of magnesium?
15. What are the postulates of Dalton's atomic theory?
16. Which of the following statements is NOT true about an atom?
(a) Atoms are the building blocks from which molecules and ions are formed.
(b) Atoms cannot exist independently.
(c) Atoms are neutral in nature
(d) Atoms combine together to form matter that we can see, feel or touch.
17. What is an atom?
18. Why is it not possible to see an atom with naked eyes?
19. Who proposed the chemical notation based on first two letters of the name of the element?
20. Name the international organization who approves names of elements.
21. What is the chemical symbol for iron?
22. Name five elements have single letter chemical symbol.
23. Name the element having following Latin names
(i) Stibium

(ii) Cuprum
(iii) Argentum
(iv) Natrium
(v) Stannum
(vi) Wolfram
(vii) plumbum
(viii) Kalium
24. Write the chemical symbols of the following:
(i) Gold
(ii) Iron
(iii) Chlorine
(iv) Mercury
25. How will you define chemical symbol?
26. What is the significance of a chemical symbol?
27. Can atoms of an element exist independently? Give examples of elements which exist in atomic form. Give examples of elements that do not exist in atomic form.
28. Why do atoms of the most of the elements not exist independently?
29. Which element has the smallest atom in size?
30. What is the atomic mass unit?
31. Magnesium is two times heavier than C-12 atom, what shall be the mass of Mg atom in terms of atomic mass units? (Given mass of C-12 atom = 12u)
32. What is relative atomic mass of an element? How it is related to atomic mass unit?
33. Define molecule. What are its important properties?
34. Based on type of substance, how molecules are classified?
35. What is atomicity?
36. Based on atomicity, how molecules are categorized?
37. Give three examples of monoatomic molecules.
38. Give four examples of diatomic molecules.
39. (i) What is the chemical formula of Water molecule? (ii) What is its atomicity? (iii) Calculate the ratio of masses of atoms of elements present in water molecule. (iv) Calculate the ratio by number of atoms of elements present in water molecule.
40. What is an ion?
41. What are polyatomic ions? Give examples?
42. Give examples of triatomic molecules.
43. What is valency of an element?
44. What is meant by the term chemical formula?
45. Write down the formulae of
(i) sodium oxide

(ii) aluminium chloride
(iii) sodium suphide
(iv) magnesium hydroxide
46. Write down the names of compounds represented by the following formulae:
(i) Al₂(SO₄)₃
(ii) CaCl₂
(iii) K2SO₄
(iv) KNO₃
(v) CaCO₃
47. Write the chemical formulae of the following. Also identify the ions present.
(a) Magnesium chloride
(b) Calcium oxide
(c) Copper nitrate
(d) Aluminium chloride
(e) Calcium carbonate.
48. Give the names of the elements present in the following compounds.
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate.
49. How many atoms are present in a (i) H₂S molecule and (ii) PO₄
3- ion?
50. (a) Write a chemical formula of a compound using zinc ion and phosphate ion. (b) Calculate the ratio by mass of atoms present in a molecule of carbon dioxide.
(Given C =12, O =16)
51. What is the molecular mass of a substance?
52. What is Formula Unit Mass? How it is different from molecular mass?
53. Calculate the formula unit masses of ZnO, Na₂O, K₂CO₃, given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u.
54. What are ionic compounds?
55. How do we know the presence of atoms if they do not exist independently for most of the elements?
56. An element 'Z' forms the following compound when it reacts with hydrogen, chlorine, oxygen and phosphorus.
ZH3, ZCl₃, Z2O₃ and ZP
(a) What is the valency of element Z?
(b) Element 'Z' is metal or non-metal?
57. Name one element each which forms diatomic and tetra atomic molecule.
58. Name one element which forms diatomic and triatomic molecule.

59. What is gram-atomic mass of an element?
60. What is gram-molecular mass of a substance?
61. Define mole. What is its significance?
62. What is molar mass?
63. Who introduced the term 'mole' in chemistry?
64. When 'mole' was chosen internationally standard way to express larger number of chemical units?
65. How many moles are there in 4.6 gms of Sodium(Na)?
66. If one mole of carbon atoms weighs 12 gram, what is the mass (in gram) of 1 atom of carbon?
67. Which has more number of atoms, 100 grams of sodium or 100 grams of iron (given, atomic mass of Na = 23 u, Fe = 56 u)?
68. 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)?
69. Convert into moles:
(a) 12 g of oxygen gas
(b) 20 g of water
(c) 22 g of carbon dioxide
70. What is the mass of: (a) 0.2 mole of oxygen atoms? (b) 0.5 mole of water molecules?
71. Find out number of atoms in 15 moles of He.
72. Calculate the number of molecules of sulphur (S8) present in 16 g of solid sulphur.
73. Calculate the number of aluminium ions present in 0.051 g of aluminium oxide.
(Hint: The mass of an ion is the same as that of an atom of the same element. Atomic mass of Al = 27 u)
74. Calculate the mass percentage of Carbon(C) , Hydrogen (H) and Oxygen (O) in one molecule of glucouse (C₆H₁₂O₆).(Atomic mass of C = 12u, H = 1u and O = 16u)
75. Calculate the number of molecules of phosphorus (P4) present in 31 gram of phosphorus.

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