Atoms and Molecules - NCERT Solution, Class 9 Science Notes - Class 9

1. 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.
Solution: 







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2. 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?
Solution: 
3.0 g of carbon combines with 8.0 g of oxygen to give 11.0 of carbon dioxide.
Thus, by the law of definite proportions, when 3.00 g of carbon is burnt in 50.00 g of oxygen, only 8.00 g of oxygen will be used to produce 11.00 gram of carbon dioxide. The remaining 42.00 g of oxygen will remain as is.

3. What are polyatomic ions? Give examples.
Solution: Polyatomic ions are those ions that contain more than one atom. These atoms can be of the same type of different type. Some examples of polyatomic ions are NO₃^-,NH₄⁺,OH^-, SO₄^2-, and SO₃^2-.

4. Write the chemical formulae of the following.
(a) Magnesium chloride
(b) Calcium oxide
(c) Copper nitrate
(d) Aluminium chloride
(e) Calcium carbonate.
Solution:
(a) MgCl₂
(b) CaO
(c) Cu(NO₃)₂
(d) AlCl₃
(e) CaCO₃

5. Give the names of the elements present in the following compounds.
(a) Quick lime
(b) Hydrogen bromide
(c) Baking powder
(d) Potassium sulphate.
Solution:
(a) Calcium and oxygen
(b) Hydrogen and bromine
(c) Sodium, hydrogen, carbon, and oxygen
(d) Potassium, sulphur, and oxygen

6. Calculate the molar mass of the following substances.
(a) Ethyne, C₂H₂
(b) Sulphur molecule, S₈
(c) Phosphorus molecule, P₄ (Atomic mass of phosphorus = 31)
(d) Hydrochloric acid, HCl
(e) Nitric acid, HNO₃
Solution:
(a) Molecular mass of ethyne, C₂H₂ = 2×12u + 2×u = 26u
(b) Molecular mass of sulphur molecule, S₈ = 8×32u = 256u
(c) Molecular mass of phosphorus molecule, P₄ = 4×31u = 124u
(d) Molecular mass of HCl = u + 35.5u = 36.5u
(e) Molecular mass of HNO₃ = u + 14u + 3×16u = 63u

7. 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)?
Solution:
(a) Mass of one mole of nitrogen atoms = molecular mass of nitrogen atoms in grams = 14 g
(b) Mass of 4 moles of aluminium atoms = 4 × molecular mass of aluminum atoms in grams = 4 × 27 = 108
(c) Mass of 10 moles of sodium sulphite = 10 × molecular mass of sodium sulphite in grams
Molecular mass of sodium sulphite, Na₂SO₃ = 2×23 + 32 + 3×16 = 126 g
Thus, the mass of 10 moles of sodium sulphite = 10 × 126 = 1260 g

8. Convert into mole.
(a) 12 g of oxygen gas
(b) 20 g of water
(c) 22 g of carbon dioxide.
Solution:








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9. What is the mass of:
(a) 0.2 mole of oxygen atoms?
(b) 0.5 mole of water molecules?
Solution:
(a) The mass of 1 mole of oxygen atoms = 16 g
Thus, the mass of 0.2 mole of oxygen atoms = 16 × 0.2 = 3.2 g
(b) The mass of 1 mole of water molecules = 18 g
Thus, the mass of 0.5 mole of water molecules = 18 × 0.5 = 9.0 g

10. Calculate the number of molecules of sulphur (S₈) present in 16 g of solid sulphur.
Solution:








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11. 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)
Solution: