Atoms and Molecules - Question Paper

ATOMS AND MOLECULES

Class 9 Science - Chapter 3

Total Marks: 80 | Time: 2.5 Hours

Instructions:

• This question paper consists of multiple choice questions, short answer questions, and long answer questions.
• All questions are compulsory.
• Use of calculators is allowed.
• Atomic mass values are given in the question or use values from periodic table.

Question Type	Number of Questions	Marks per Question	Total Marks
Multiple Choice Questions (MCQ)	10	1	10
Short Answer Questions - Type I	8	2	16
Short Answer Questions - Type II	6	3	18
Long Answer Questions	4	9	36
TOTAL			80

Section A: Multiple Choice Questions (1 Mark Each)

1 Mark

Q1.

According to Dalton's atomic theory, atoms are:

(a) Divisible particles

(b) Indivisible particles that cannot be created or destroyed

(c) Charged particles

(d) Made of smaller sub-atomic particles

1 Mark

Q2.

The law of conservation of mass states that:

(a) Mass can be created but not destroyed

(b) Mass can neither be created nor destroyed in a chemical reaction

(c) Mass always decreases in a reaction

(d) Mass always increases in a reaction

1 Mark

Q3.

The atomic mass unit is defined as:

(a) 1/16 of the mass of oxygen atom

(b) 1/12 of the mass of carbon-12 atom

(c) 1/8 of the mass of oxygen atom

(d) Mass of one hydrogen atom

1 Mark

Q4.

Ozone (O₃) is formed when three oxygen atoms combine. The atomicity of ozone is:

(a) 1

(b) 2

(c) 3

(d) 4

1 Mark

Q5.

Which of the following is a polyatomic ion?

(a) Na⁺

(b) Cl⁻

(c) NO₃⁻

(d) O²⁻

1 Mark

Q6.

The symbol of an element as per IUPAC rules:

(a) Has both letters in uppercase

(b) Has both letters in lowercase

(c) Has first letter uppercase and second lowercase

(d) Can be written in any format

1 Mark

Q7.

The valency of calcium is:

(a) 1

(b) 2

(c) 3

(d) 4

1 Mark

Q8.

In the formula Ca(OH)₂, the brackets are used because:

(a) Calcium is a metal

(b) There are two OH groups in the compound

(c) Oxygen is more electronegative

(d) It makes the formula shorter

1 Mark

Q9.

The molecular mass of CO₂ (C=12, O=16) is:

(a) 28 u

(b) 32 u

(c) 44 u

(d) 60 u

1 Mark

Q10.

Which statement is true about molecules?

(a) Molecules can exist independently only for atoms

(b) Molecules are always made of more than one atom

(c) Molecules are the smallest particle capable of independent existence

(d) Molecules cannot be broken down further

Section B: Short Answer Questions - Type I (2 Marks Each)

2 Marks

Q11.

State the law of constant proportions. Give an example.

2 Marks

Q12.

Differentiate between an atom and a molecule.

2 Marks

Q13.

What is atomicity? Give examples of monoatomic, diatomic, and polyatomic molecules.

2 Marks

Q14.

Why are atoms of most elements unable to exist independently?

2 Marks

Q15.

What are cations and anions? Give one example of each.

2 Marks

Q16.

Define valency. How does it help in writing chemical formulae?

2 Marks

Q17.

Calculate the atomic mass ratio of carbon to oxygen given that in carbon monoxide (CO), 3 g of carbon combines with 4 g of oxygen.

2 Marks

Q18.

What do you understand by the term 'formula unit mass'? How is it different from molecular mass?

Section C: Short Answer Questions - Type II (3 Marks Each)

3 Marks

Q19.

State Dalton's atomic theory. Which of the postulates explains the law of conservation of mass?

3 Marks

Q20.

Write the chemical formulae of the following compounds:

(a) Magnesium hydroxide
(b) Sodium carbonate
(c) Ammonium sulphate

3 Marks

Q21.

Calculate the molecular masses of:

(a) H₂O (H=1, O=16)
(b) NH₃ (N=14, H=1)
(c) CH₄ (C=12, H=1)

3 Marks

Q22.

In a chemical reaction, 2.4 g of carbon reacted with 6.4 g of oxygen to form 8.8 g of carbon dioxide. Verify that the mass is conserved in this reaction.

3 Marks

Q23.

Write the names of the elements present in the following compounds:

(a) Calcium carbonate (CaCO₃)
(b) Potassium sulphate (K₂SO₄)
(c) Copper nitrate (Cu(NO₃)₂)

3 Marks

Q24.

Calculate the formula unit mass of:

(a) NaCl (Na=23, Cl=35.5)
(b) CaCl₂ (Ca=40, Cl=35.5)
(c) Al₂O₃ (Al=27, O=16)

Section D: Long Answer Questions (9 Marks Each)

9 Marks

Q25.

(a) Explain the law of conservation of mass with a suitable example and a chemical equation.

(b) In a reaction, 5.3 g of sodium carbonate reacted with 6 g of acetic acid. The products were 2.2 g of carbon dioxide, 0.9 g of water, and 8.2 g of sodium acetate. Show that these observations agree with the law of conservation of mass. (5+4)

9 Marks

Q26.

(a) Explain the postulates of Dalton's atomic theory that account for the law of constant proportions.

(b) State the postulate of Dalton's theory that explains the law of conservation of mass.

(c) Give the limitations of Dalton's atomic theory. (4+2+3)

9 Marks

Q27.

(a) Explain what is meant by atomicity. How does it differ from valency?

(b) Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. What mass of oxygen gas would be required to react completely with 3 g of hydrogen gas?

(c) Find the ratio by number of atoms in CO₂, given that C:O mass ratio is 3:8 and atomic masses are C=12, O=16. (3+3+3)

9 Marks

Q28.

(a) What is an ion? Differentiate between cations and anions with examples.

(b) What are polyatomic ions? Give four examples with their charges.

(c) Using the valencies given in the table, write the chemical formulae of:
i) Aluminium sulphate
ii) Magnesium nitrate
iii) Iron (III) hydroxide (3+3+3)

ANSWER KEY WITH EXPLANATIONS

Section A: Multiple Choice Questions

Q1. Answer: (b)

Indivisible particles that cannot be created or destroyed

Explanation: According to Dalton's atomic theory, atoms are the smallest indivisible particles of matter. They cannot be created, destroyed, or divided further in chemical reactions.

Q2. Answer: (b)

Mass can neither be created nor destroyed in a chemical reaction

Explanation: The law of conservation of mass states that during a chemical reaction, the total mass of reactants equals the total mass of products. This law was established by Lavoisier through careful experiments.

Q3. Answer: (b)

1/12 of the mass of carbon-12 atom

Explanation: In 1961, the atomic mass unit (u) was standardized using carbon-12 as the reference. One atomic mass unit is exactly 1/12th the mass of a carbon-12 atom. This provides a universal standard for measuring atomic masses.

Q4. Answer: (c)

3

Explanation: Atomicity is the number of atoms present in a molecule. Ozone (O₃) contains 3 oxygen atoms, so its atomicity is 3. It is a tri-atomic or polyatomic molecule.

Q5. Answer: (c)

NO₃⁻ (Nitrate ion)

Explanation: A polyatomic ion is a group of atoms carrying a net charge. NO₃⁻ is composed of nitrogen and three oxygen atoms carrying a negative charge. Na⁺, Cl⁻, and O²⁻ are monoatomic ions (single atoms with charges).

Q6. Answer: (c)

Has first letter uppercase and second lowercase

Explanation: According to IUPAC rules, element symbols consist of one or two letters. The first letter is always written in uppercase and the second letter (if present) is written in lowercase. Example: Al (Aluminium), Co (Cobalt), Ca (Calcium).

Q7. Answer: (b)

2

Explanation: Calcium is an alkaline earth metal with atomic number 20. It has 2 valence electrons, so its valency is 2. This means calcium forms compounds like CaO, CaCl₂, etc., where it combines with elements using its 2-electron capacity.

Q8. Answer: (b)

There are two OH groups in the compound

Explanation: Brackets are used in chemical formulae when there are two or more of the same polyatomic ion. In Ca(OH)₂, there are two hydroxide (OH⁻) groups bonded to one calcium ion. Without brackets, it would be ambiguous. The subscript 2 outside the bracket indicates two OH groups.

Q9. Answer: (c)

44 u

Explanation: Molecular mass of CO₂ = Atomic mass of C + (2 × Atomic mass of O) = 12 + (2 × 16) = 12 + 32 = 44 u. The subscript 2 indicates two oxygen atoms in each molecule.

Q10. Answer: (c)

Molecules are the smallest particle capable of independent existence

Explanation: A molecule is defined as the smallest particle of an element or compound that can exist independently and still show all the properties of that substance. While atoms are components of molecules, most atoms cannot exist independently under normal conditions.

Section B: Short Answer Questions - Type I (2 Marks)

Q11. Law of Constant Proportions

Definition: In a pure chemical compound, the elements are always present in definite proportions by mass, irrespective of the source of the compound or the method of preparation.

Example: In water (H₂O), hydrogen and oxygen are always present in the ratio of 1:8 by mass. Whether water comes from a river, rain, or is prepared in the laboratory, the mass ratio remains 1:8. If 18 g of water is decomposed, it always yields 2 g of hydrogen and 16 g of oxygen.

Marks Distribution: Definition (1 mark) + Example (1 mark)

Q12. Difference between Atom and Molecule

Atom	Molecule
The smallest particle of an element	The smallest particle of an element or compound
Cannot usually exist independently under normal conditions	Can exist independently under normal conditions
Does not show properties of the element	Shows all properties of the substance
Made of nucleus and electrons	Made of two or more atoms bonded together

Marks Distribution: Any 4 points with explanations (2 marks)

Q13. Atomicity and Examples

Definition: Atomicity is the number of atoms present in a molecule of an element or compound.

Examples:
• Monoatomic (Atomicity = 1): Argon (Ar), Helium (He), Neon (Ne) - Noble gases with single atoms
• Diatomic (Atomicity = 2): Oxygen (O₂), Nitrogen (N₂), Hydrogen (H₂), Chlorine (Cl₂)
• Polyatomic (Atomicity > 2): Ozone (O₃), Phosphorus (P₄), Sulphur (S₈), Water (H₂O)

Marks Distribution: Definition (1 mark) + Examples (3 different types with 1 mark each)

Q14. Why Atoms Cannot Exist Independently

Most atoms of elements are not able to exist independently under normal conditions because:

1. Incomplete electron configuration: Most atoms have incomplete valence shells and need additional electrons to achieve stability (octet rule for non-metals, or duplet for hydrogen).

2. High reactivity: Isolated atoms are extremely reactive and tend to combine with other atoms to fill their valence shells.

3. Formation of molecules/ions: Atoms combine with each other through chemical bonding to form stable molecules or ions, which can then exist independently under normal conditions.

Example: Oxygen atoms (O) cannot exist alone; they combine in pairs to form oxygen molecules (O₂) which are stable and can exist independently.

Marks Distribution: Any 2 valid reasons with explanation (1 mark each)

Q15. Cations and Anions

Definition:
• Cations: Positively charged ions formed by the loss of electrons from atoms. Example: Na⁺ (Sodium ion), Ca²⁺ (Calcium ion), H⁺ (Hydrogen ion)

• Anions: Negatively charged ions formed by the gain of electrons by atoms. Example: Cl⁻ (Chloride ion), O²⁻ (Oxide ion), OH⁻ (Hydroxide ion)

In ionic compounds, cations and anions are held together by electrostatic forces (ionic bonds), resulting in neutral compounds.

Marks Distribution: Definition of cations (0.5 marks) + Example (0.5 marks) + Definition of anions (0.5 marks) + Example (0.5 marks)

Q16. Valency and Its Role

Definition: Valency is the combining power or capacity of an element. It represents the number of electrons an atom can lose, gain, or share to complete its valence shell and achieve stability.

Role in Writing Chemical Formulae:
1. Valency indicates how many atoms of another element an atom can combine with.
2. To write a chemical formula, we cross the valencies of combining elements.
3. The valencies become the subscripts in the formula.
4. Example: For NaCl, Na has valency +1 and Cl has valency -1. Criss-crossing gives NaCl (neutral compound).
5. For CaCl₂, Ca has valency +2 and Cl has valency -1. Criss-crossing: Ca²Cl₁ → CaCl₂

Marks Distribution: Definition (1 mark) + Role with example (1 mark)

Q17. Atomic Mass Ratio

Given: In carbon monoxide (CO): 3 g of carbon combines with 4 g of oxygen

Solution:
Mass of carbon = 3 g
Mass of oxygen = 4 g

Since atoms combine in whole number ratios and one atom of each element is present in CO:
1 atom of C : 1 atom of O
Mass of C : Mass of O = 3 : 4

Therefore, the atomic mass ratio of C to O = 3:4

Or, we can say: Atomic mass of C : Atomic mass of O = 3 : 4
If we take atomic mass of O = 16, then atomic mass of C = 12 (which matches modern atomic masses)

Marks Distribution: Setting up the ratio (1 mark) + Correct answer (1 mark)

Q18. Formula Unit Mass vs Molecular Mass

Formula Unit Mass: The sum of atomic masses of all atoms in a formula unit of an ionic compound. The term "formula unit" is used because ionic compounds do not form discrete molecules but exist as a continuous arrangement of cations and anions. Example: NaCl has a formula unit mass = 23 + 35.5 = 58.5 u

Molecular Mass: The sum of atomic masses of all atoms in a molecule of a covalent compound. The term "molecular mass" is used for compounds that form discrete molecules. Example: H₂O has a molecular mass = (2 × 1) + 16 = 18 u

Difference: The concept is similar - both represent the total mass of the formula/molecule. However, the terminology differs: "molecular mass" for covalent compounds with molecules, and "formula unit mass" for ionic compounds without discrete molecules.

Marks Distribution: Definition of formula unit mass (1 mark) + Definition of molecular mass (1 mark)

Section C: Short Answer Questions - Type II (3 Marks)

Q19. Dalton's Atomic Theory

Postulates of Dalton's Atomic Theory:
1. All matter is made up of very tiny particles called atoms that participate in chemical reactions.
2. Atoms are indivisible and indestructible particles that cannot be created or destroyed in a chemical reaction.
3. Atoms of a given element are identical in mass and chemical properties.
4. Atoms of different elements have different masses and chemical properties.
5. Atoms combine in the ratio of small whole numbers to form compounds.
6. The relative number and kinds of atoms are constant in a given compound.

Postulate explaining Law of Conservation of Mass: The second postulate - "Atoms are indivisible and indestructible particles that cannot be created or destroyed in a chemical reaction" - explains the law of conservation of mass. Since atoms cannot be created or destroyed, only rearranged during chemical reactions, the total mass of reactants equals the total mass of products.

Marks Distribution: Listing postulates (1-1.5 marks) + Identification of relevant postulate (1-1.5 marks)

Q20. Chemical Formulae

(a) Magnesium hydroxide:
Mg²⁺ and OH⁻
Criss-cross valencies: Mg²⁺ requires 2 OH⁻ groups
Formula: Mg(OH)₂

(b) Sodium carbonate:
Na⁺ and CO₃²⁻
Criss-cross valencies: 2 Na⁺ required for 1 CO₃²⁻
Formula: Na₂CO₃

(c) Ammonium sulphate:
NH₄⁺ and SO₄²⁻
Criss-cross valencies: 2 NH₄⁺ required for 1 SO₄²⁻
Formula: (NH₄)₂SO₄

Marks Distribution: Each formula (1 mark each)

Q21. Molecular Masses

(a) H₂O (Water):
Atomic mass of H = 1 u, O = 16 u
Molecular mass = (2 × 1) + 16 = 2 + 16 = 18 u

(b) NH₃ (Ammonia):
Atomic mass of N = 14 u, H = 1 u
Molecular mass = 14 + (3 × 1) = 14 + 3 = 17 u

(c) CH₄ (Methane):
Atomic mass of C = 12 u, H = 1 u
Molecular mass = 12 + (4 × 1) = 12 + 4 = 16 u

Marks Distribution: Each calculation (1 mark each) - Formula setup, Substitution, and correct answer

Q22. Verification of Law of Conservation of Mass

Given:
Carbon (C) = 2.4 g
Oxygen (O₂) = 6.4 g
Carbon dioxide (CO₂) formed = 8.8 g

Reaction: C + O₂ → CO₂

Solution:
Total mass of reactants = Mass of C + Mass of O₂
= 2.4 + 6.4
= 8.8 g

Total mass of products = Mass of CO₂
= 8.8 g

Verification:
Mass of reactants = Mass of products
8.8 g = 8.8 g ✓

Therefore, the law of conservation of mass is verified. The total mass is conserved in the reaction.

Marks Distribution: Writing equation (0.5 marks) + Calculating reactants (1 mark) + Comparing masses (1 mark) + Conclusion (0.5 marks)

Q23. Names of Elements in Compounds

(a) Calcium carbonate (CaCO₃):
Elements present: Calcium (Ca), Carbon (C), Oxygen (O)

(b) Potassium sulphate (K₂SO₄):
Elements present: Potassium (K), Sulphur (S), Oxygen (O)

(c) Copper nitrate (Cu(NO₃)₂):
Elements present: Copper (Cu), Nitrogen (N), Oxygen (O)

Note: When analyzing compound formulae, identify each element symbol and then write the element name corresponding to that symbol.

Marks Distribution: Each compound (1 mark for identifying all 3 elements correctly)

Q24. Formula Unit Masses

(a) NaCl:
Formula unit mass = Atomic mass of Na + Atomic mass of Cl
= 23 + 35.5
= 58.5 u

(b) CaCl₂:
Formula unit mass = Atomic mass of Ca + (2 × Atomic mass of Cl)
= 40 + (2 × 35.5)
= 40 + 71
= 111 u

(c) Al₂O₃:
Formula unit mass = (2 × Atomic mass of Al) + (3 × Atomic mass of O)
= (2 × 27) + (3 × 16)
= 54 + 48
= 102 u

Marks Distribution: Each calculation (1 mark each) - Formula setup, Substitution, and correct answer

Section D: Long Answer Questions (9 Marks)

Q25. Law of Conservation of Mass

(a) Explanation with Example:

Law of Conservation of Mass: During a chemical reaction, matter cannot be created or destroyed. The total mass of reactants (initial substances) is always equal to the total mass of products (final substances formed).

Example: Consider the combustion of carbon:

Chemical Equation: C + O₂ → CO₂

If 12 g of carbon reacts with 32 g of oxygen:
Mass of reactants = 12 g (C) + 32 g (O₂) = 44 g
Mass of product = 44 g (CO₂)

The mass is conserved - no mass is lost or gained during the reaction.

(b) Verification for Given Reaction:

Reaction: Na₂CO₃ + 2CH₃COOH → 2CH₃COONa + CO₂ + H₂O

Given data:
Sodium carbonate (Na₂CO₃) = 5.3 g
Acetic acid (CH₃COOH) = 6 g
Products: CO₂ = 2.2 g, H₂O = 0.9 g, Sodium acetate (CH₃COONa) = 8.2 g

Verification:
Total mass of reactants = 5.3 + 6 = 11.3 g
Total mass of products = 2.2 + 0.9 + 8.2 = 11.3 g

Mass of reactants = Mass of products = 11.3 g

✓ The observations agree with the law of conservation of mass.

Marks Distribution: (a) Definition (2 marks) + Example with equation and calculation (3 marks) = 5 marks | (b) Setup (1 mark) + Calculation (2 marks) + Conclusion (1 mark) = 4 marks

Q25. Dalton's Atomic Theory

(a) Postulates Explaining Law of Constant Proportions:

The law of constant proportions states that in a pure chemical compound, elements are always present in the same proportion by mass. The postulates of Dalton's theory that explain this law are:

Postulate 5: "Atoms combine in the ratio of small whole numbers to form compounds."
This explains why compounds always have the same composition. The elements always combine in the same small whole number ratio (e.g., H:O = 2:1 in water).

Postulate 6: "The relative number and kinds of atoms are constant in a given compound."
This directly states that every molecule of a compound has the same atoms in the same proportions.

Example: In water (H₂O), every molecule contains 2 hydrogen atoms and 1 oxygen atom. Therefore, the mass ratio is always 1:8 (H:O), regardless of the source.

(b) Postulate Explaining Law of Conservation of Mass:

Postulate 2: "Atoms are indivisible particles which cannot be created or destroyed in a chemical reaction."

Since atoms cannot be created or destroyed, only rearranged during a chemical reaction, the total number an