Chapter Exam

Chapter 3: Atoms and Molecules — Online MCQ Test

Q: What is the chemical formula of water?

Answer: A. H2O — A water molecule contains two hydrogen atoms and one oxygen atom, so its formula is H2O.

Q: Which law states that the total mass of reactants equals the total mass of products in a chemical reaction?

Answer: B. Law of conservation of mass — The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction.

Q: How many moles of atoms are present in 88 g of CO2?

Answer: A. 1 mole of molecules and 3 moles of atoms — The molar mass of CO2 is 44 g/mol, so 88 g is 2 moles of CO2 molecules. Each molecule has 3 atoms, so total atoms = 6 moles of atoms. The correct option is D.

SCIENCE · CLASS 9th · Karnataka State Board

Practice Chapter 3: Atoms and Molecules with a free chapter-wise online MCQ test for Karnataka State Board CLASS 9th SCIENCE. This chapter covers: This chapter covers laws of chemical combination Dalton atomic theory atoms molecules atomic mass molecular mass chemical formulas and mole concept.. AI-generated questions from basic to board-exam level, with instant results and explanations.

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Chapter 3: Atoms and Molecules — Important Questions & Answers (FAQ)

Frequently asked questions from Karnataka State Board CLASS 9th SCIENCE — Chapter 3: Atoms and Molecules, with answers and explanations. These are sample questions; the exam has its own separate question set.

Which of the following is a postulate of Dalton's atomic theory?

A. Atoms can be created and destroyed in chemical reactions
B. Atoms combine in fixed, simple whole-number ratios to form compounds ✓
C. All atoms of an element have different masses
D. Atoms are divisible into smaller particles in chemical reactions

Answer: B. Atoms combine in fixed, simple whole-number ratios to form compounds
Dalton stated that atoms combine in fixed whole-number ratios to form compounds. This explains the definite composition of substances.

What is the chemical formula of water?

A. H2O ✓
B. HO2
C. H2O2
D. OH2

Answer: A. H2O
A water molecule contains two hydrogen atoms and one oxygen atom, so its formula is H2O.

Which law states that the total mass of reactants equals the total mass of products in a chemical reaction?

A. Law of multiple proportions
B. Law of conservation of mass ✓
C. Law of constant composition
D. Law of reciprocal proportions

Answer: B. Law of conservation of mass
The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction.

Which of the following supports the law of constant proportions?

A. Water always contains hydrogen and oxygen in the ratio 2:1 by atoms ✓
B. Water can contain any amount of hydrogen and oxygen
C. Hydrogen and oxygen combine in variable mass ratios in water
D. Water molecules do not have fixed composition

Answer: A. Water always contains hydrogen and oxygen in the ratio 2:1 by atoms
A compound always has the same elements in the same fixed ratio. In water, hydrogen and oxygen are present in a fixed proportion.

How many moles of atoms are present in 88 g of CO2?

A. 1 mole of molecules and 3 moles of atoms ✓
B. 2 moles of molecules and 2 moles of atoms
C. 1 mole of molecules and 1 mole of atoms
D. 2 moles of molecules and 6 moles of atoms

Answer: A. 1 mole of molecules and 3 moles of atoms
The molar mass of CO2 is 44 g/mol, so 88 g is 2 moles of CO2 molecules. Each molecule has 3 atoms, so total atoms = 6 moles of atoms. The correct option is D.