Test 4

 

Here are 20 MCQs on Average Atomic Mass for Class 9, following NCERT style.
Answer key is given at the end.

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20 MCQs – Average Atomic Mass (Class 9)

1. Average atomic mass depends on which factor?

a) Number of protons only
b) Number of neutrons only
c) Percentage abundance of isotopes
d) Valency of element

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2. Chlorine has two isotopes: Cl-35 (75%) and Cl-37 (25%). Its average atomic mass is closest to—

a) 35.5 u
b) 36.5 u
c) 34.5 u
d) 35.0 u

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3. What are isotopes?

a) Atoms with same mass but different atomic numbers
b) Atoms with same atomic number but different mass numbers
c) Atoms that are identical
d) Atoms with same number of neutrons

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4. The average atomic mass of an element having only one isotope will be—

a) equal to mass number of isotope
b) double the mass number
c) half of mass number
d) cannot be calculated

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5. Boron has two isotopes: B-10 (20%) and B-11 (80%). Average atomic mass = ?

a) 10.2 u
b) 10.8 u
c) 11.2 u
d) 11.8 u

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6. Which element’s average atomic mass is exactly equal to the mass of one of its isotopes?

a) Hydrogen
b) Sodium
c) Fluorine
d) None of these

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7. If an element X has isotopes X-20 (30%) and X-22 (70%), the average atomic mass is—

a) 20.6 u
b) 21.4 u
c) 22.0 u
d) 21.0 u

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8. Isotopes of an element have—

a) same chemical properties
b) different chemical properties
c) same number of neutrons
d) different atomic numbers

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9. The unit of average atomic mass is—

a) kilogram
b) gram
c) u
d) mol

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10. Why does chlorine show atomic mass 35.5 u?

a) It has fractional protons
b) It has isotopes in different ratios
c) It has unstable electrons
d) It has different ions

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11. Which of the following can have different mass numbers?

a) Isotopes
b) Atoms in molecule
c) Ions
d) Electrons

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12. Hydrogen has isotopes H-1, H-2 and H-3. Which isotope is most abundant?

a) H-1
b) H-2
c) H-3
d) All are equal

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13. The average atomic mass of magnesium is 24.3 u. This indicates—

a) Mg has only one isotope
b) Mg has three isotopes in different percentages
c) Mg has fractional protons
d) Mg is unstable

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14. The average atomic mass of element Y is closer to 30 u. Its isotopes are Y-28 and Y-30. Which is more abundant?

a) Y-28
b) Y-30
c) Both equal
d) Cannot be said

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15. An element Z has isotopes Z-100 (10%) and Z-120 (90%). Average atomic mass = ?

a) 102 u
b) 110 u
c) 118 u
d) 120 u

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16. If two isotopes have equal percentage abundance, average atomic mass =

a) lower isotope mass
b) higher isotope mass
c) simple average of both masses
d) cannot be found

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17. The term “relative atomic mass” refers to—

a) mass of atom compared with carbon-12
b) mass of atom in kilograms
c) mass of atom in grams
d) mass of nucleus only

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18. Which statement is true about isotopes of the same element?

a) Same number of electrons
b) Different number of protons
c) Different atomic numbers
d) Different chemical properties

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19. If element A has isotopes A-40 (50%) and A-42 (50%), average atomic mass is—

a) 41 u
b) 40 u
c) 42 u
d) 41.5 u

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20. Average atomic mass is the weighted average of—

a) ions
b) isotopes
c) compounds
d) electrons

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Answer Key

1. c
2. a
3. b
4. a
5. b
6. a
7. b
8. a
9. c
10. b
11. a
12. a
13. b
14. b
15. c
16. c
17. a
18. a
19. a
20. b

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If you want, I can also create numerical questions or level-wise practice worksheets.