Check NCERT based extra questions for CBSE Class 9 Science Chapter 3 - Atoms and Molecules. Accurate answers have been provided for all the questions. All the questions are based on important concepts. These extra questions are important for self-assessment and prepare effectively for your exams. You can read and download all the questions and answers to use them for your exam preparations.
CBSE Class 9 Science Extra Questions for Chapter 3 - Atoms and Molecules:
(a) Law of Conservation of mass
(b) Law of constant proportion
(a) It states that mass can neither be created nor destroyed in a chemical reaction.
(b) Law of constant proportion states that in a chemical compound the elements are always present in a definite proportion by mass.
2. What do you mean by atomicity? What is the atomicity of ozone?
The number of atoms constituting a molecule is known as its atomicity.
Ozone (O3) contains 3 oxygen atoms. So its atomicity is 3.
3. Calculate the mass of 1 atom of Hydrogen.
One mole of hydrogen weighs = 1 g
∴ 6.02×1023 atoms of hydrogen weigh = 1/ (6.02×1023)
∴ 1 atom of hydrogen weighs = 1.66 ×10−24 g
4. Name a non-metal which is polyatomic.
Sulphur (S) called polyatomic because each sulphur molecule (S8) is made up of 8 sulphur atoms.
5. Calculate the mass percentage of oxygen present in HNO3.
The molecular mass of HNO3 = the atomic mass of H + the atomic mass of N + 3 × the atomic mass of O = 1 + 14 + 48 = 63 g
Mass percentage of oxygen = (Mass of O in compound/ molecular mass) x 100
= (48/63) x 100 = 76.19%
6. What is Avogadro constant? Write its value.
The Avogadro constant is defined as the number of atoms in exactly 12 g of carbon-12.
Avogadro constant or Avogadro number = 6.022 × 1023
7. What do you mean by one mole of a substance?
The mole is the amount of substance that contains the same number of particles (atoms/ ions/ molecules/ formula units etc.) as there are atoms in exactly 12 g of carbon-12.
8. How many molecules of water are present in 36 grams of water?
Molecular mass of water (H2O) = 2(H)+1(O) = 2×1 + 1×16 = 2 + 16 = 18 gm
Thus, 1 mole of water weighs= 18 g
Or 18g of water contain 1 mole, i.e., 6.023 x 1023 molecules of water.
36 gm of water contain = (36/18) x 6.023 x 1023
= 2 x 6.023 x 1023 = 12.044 x 1023molecules of water
9. Which has more number of atoms: 100g of N2 or 100g of NH3?
NH₃ has more atoms than N₂.
10. A group of atoms carrying a fixed charge on them is called _______.
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