9th Standard Chapter 3 Atoms and Molecules

Chemistry Exercises and Solutions

Chemistry Exercises and Solutions

1. Percentage Composition of Boron and Oxygen

Question: A 0.240 g sample of a compound Y contains 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage by mass of boron and oxygen in compound Y.

Answer:

  1. % B = (0.096 g / 0.240 g) × 100 = 40.0 %
  2. % O = (0.144 g / 0.240 g) × 100 = 60.0 %

2. Mass of CO₂ Formed and Governing Law

Question:
When 3.00 g of carbon burns in 8.00 g of oxygen, 11.00 g of carbon dioxide is formed.
a) How many grams of CO₂ are formed when 3.00 g of carbon burns in 50.00 g of oxygen?
b) Which law of chemical combination is illustrated by this observation?

Answer:

  1. Moles of C = 3.00 g / 12.01 g·mol⁻¹ ≈ 0.250 mol
  2. Required O₂ = 0.250 mol × 32.00 g·mol⁻¹ = 8.00 g → C is limiting, O₂ in excess
  3. Moles of CO₂ = 0.250 mol → Mass = 0.250 mol × 44.01 g·mol⁻¹ = 11.00 g

Law Illustrated: Law of Definite Proportions (Constant Composition)

3. Polyatomic Ions

Question: What is a polyatomic ion? Give two examples.

Answer:

A polyatomic ion is a charged species composed of two or more atoms covalently bonded that behaves as a single unit. Examples:

  • Sulfate, SO₄²⁻
  • Nitrate, NO₃⁻

4. Writing Chemical Formulae

Question: Write the chemical formulae for each of the following compounds:

  • a) Magnesium chloride
  • b) Calcium oxide
  • c) Copper(II) nitrate
  • d) Aluminium chloride
  • e) Calcium carbonate

Answer:

  • a) MgCl₂
  • b) CaO
  • c) Cu(NO₃)₂
  • d) AlCl₃
  • e) CaCO₃

5. Elements Present in Compounds

Question: Identify the elements present in each of the following substances:

  • a) Quick lime (CaO)
  • b) Hydrogen bromide (HBr)
  • c) Baking powder (commonly NaHCO₃)
  • d) Potassium sulfate (K₂SO₄)

Answer:

  • a) Ca, O
  • b) H, Br
  • c) Na, H, C, O
  • d) K, S, O

6. Molar Mass Calculations

Question: Calculate the molar mass of each of the following substances:

  • a) Ethyne (C₂H₂)
  • b) Sulfur molecule, S₈
  • c) Phosphorus molecule, P₄
  • d) Hydrochloric acid (HCl)
  • e) Nitric acid (HNO₃)

Answer:

  • a) C₂H₂: 2×12.01 + 2×1.008 = 26.04 g·mol⁻¹
  • b) S₈: 8×32.06 = 256.48 g·mol⁻¹
  • c) P₄: 4×30.97 = 123.88 g·mol⁻¹
  • d) HCl: 1.008 + 35.45 = 36.46 g·mol⁻¹
  • e) HNO₃: 1.008 + 14.01 + 3×16.00 = 63.02 g·mol⁻¹

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