9th Class (CBSE) Science: Atoms and Molecules

Question: 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.

Answer: Boron and oxygen compound —> Boron + Oxygen
0.24 g —> 0.096 g + 0.144 g

Question: 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?

Answer: The reaction of burning of carbon in oxygen may be written as:

It shows that 12 g of carbon bums in 32 g oxygen to form 44 g of carbon dioxide. Therefore 3 g of carbon reacts with 8 g of oxygen to form 11 g of carbon dioxide. It is given that 3.0 g of carbon is burnt with 8 g of oxygen to produce 11.0 g of CO₂. Consequently 11.0 g of carbon dioxide will be formed when 3.0 g of C is burnt in 50 g of oxygen consuming 8 g of oxygen, leaving behind 50 – 8 = 42 g of O₂. The answer governs the law of constant proportion.

Question: What are poly atomic ions? Give examples.

Answer: The ions which contain more than one atoms (same kind or may be of different kind) and behave as a single unit are called polyatomic ions e.g., OH^–, SO₄^2-, CO₃^2-.

Question: Write the chemical formulae of the following:
(1) Magnesium chloride
(2) Calcium oxide
(3) Copper nitrate
(4) Aluminium chloride
(5) Calcium carbonate.

Answer:

1. Magnesium chloride
   Symbol —> Mg Cl
   Change —> +2 -1
   Formula —> MgCl₂
2. Calcium oxide
   Symbol —> Ca O
   Charge —> +2 -2
   Formula —> CaO
3. Copper nitrate
   Symbol —> Cu NO
   Change +2 -1
   Formula -4 CU(N0₃)₂
4. Aluminium chloride
   Symbol —> Al Cl
   Change —> +3 -1
   Formula —> AlCl₃
5. Calcium carbonate
   Symbol —> Ca CO₃
   Change —> +2 -2
   Formula —> CaC0₃

Question: Give the names of the elements present in the following compounds:
(1) Quick lime
(2) Hydrogen bromide
(3) Baking powder
(4) Potassium sulphate.

Answer:

1. Quick lime —> Calcium oxide
   Elements —> Calcium and oxygen
2. Hydrogen bromide
   Elements —> Hydrogen and bromine
3. Baking powder —> Sodium hydrogen carbonate
   Elements —> Sodium, hydrogen, carbon and oxygen
4. Potassium sulphate

Elements —> Potassium, sulphur and oxygen

Question: Calculate the molar mass of the following substances.
(1) Ethyne, C₂H₂
(2) Sulphur molecule, S₈
(3) Phosphorus molecule, P₄ (Atomic mass of phosphorus = 31)
(4) Hydrochloric acid, HCl
(5) Nitric acid, HNO₃

Answer: The molar mass of the following: [Unit is ‘g’]

1. Ethyne, C₂H₂ = 2 x 12 + 2 x 1 = 24 + 2 = 26 g
2. Sulphur molecule, S₈ = 8 x 32 = 256 g
3. Phosphorus molecule, P₄=4 x 31 = i24g
4. Hydrochloric acid, HCl = 1 x 1 + 1 x 35.5 = 1 + 35.5 = 36.5 g
5. Nitric acid, HN0₃ = 1 x 1 + 1 x 14 + 3 x 16 = 1 + 14 + 48 = 63 g

Question: What is the mass of

1. 1 mole of nitrogen atoms?
2. 4 moles of aluminium atoms (Atomic mass of aluminium = 27)?
3. 10 moles of sodium sulphite (Na₂S0₃)?

Answer:

1. Mass of 1 mole of nitrogen atoms = 14 g
2. 4 moles of aluminium atoms
   Mass of 1 mole of aluminium atoms = 27 g
   ∴ Mass of 4 moles of aluminium atoms = 27 x 4 = 108 g
3. 10 moles of sodium sulphite (Na₂SO₃)
   Mass of 1 mole of Na₂SO₃ = 2 x 23 + 32 + 3 x 16 = 46 + 32 + 48 = 126 g
   ∴ Mass of 10 moles of Na₂SO₃ = 126 x 10 = 1260 g

Question: Convert into mole.

1. 12 g of oxygen gas
2. 20 g of water
3. 22 g of Carbon dioxide.

Answer:

1. Given mass of oxygen gas = 12 g
   Molar mass of oxygen gas (O₂) = 32 g
   Mole of oxygen gas 12/32 = 0.375 mole
2. Given mass of water = 20 g
   Molar mass of water (H₂O) = (2 x 1) + 16 = 18 g
   Mole of water = 20/18 = 1.12 mole
3. Given mass of Carbon dioxide = 22 g
   Molar mass of carbon dioxide (CO₂) = (1 x 12) + (2 x 16)
   = 12 + 32 = 44 g
   ∴ Mole of carbon dioxide = 22/44 = 0.5 mole