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CBSE Class 9  >  Class 9 Notes  >  Science   >  Unit Test (Solutions): Atoms and Molecules

Unit Test (Solutions): Atoms and Molecules

Time: 1 hour
M.M. 30
Attempt all questions. 
Questions 1 to 5 carry 1 mark each. 
Questions 6 to 8 carry 2 marks each. 
Questions 9 to 11 carry 3 marks each. 
Questions 12 & 13 carry 5 marks each.

Q1. Who proposed the concept of indivisible particles called atoms? (1 Mark)

Ans: Democritus

Q2. Define an atomic mass unit. (1 Mark)

Ans: One atomic mass unit (u) is a mass unit equal to exactly one-twelfth (1/12th) the mass of one carbon-12 atom.

Q3. What is the valency of the sodium ion? (1 Mark)

Ans: 1

Q4. Which law states that elements in a compound are present in definite proportions by mass? (1 Mark)

Ans: Law of Definite Proportions

Q5. What is the atomicity of the oxygen molecule (O2)? (1 Mark)

Ans: Diatomic

Q6. State the Law of Conservation of Mass with an example. (2 Marks)

Ans: Mass is neither created nor destroyed in a chemical reaction. Example: In the reaction of sodium carbonate (5.3 g) with acetic acid (6 g), the products (carbon dioxide 2.2 g, water 0.9 g, sodium acetate 8.2 g) have a total mass of 11.3 g, equal to the reactants' mass.
sodium carbonate + acetic acid → sodium acetate + carbon dioxide + water

Q7. Differentiate between an atom and a molecule. (2 Marks)

Ans:

  • An atom is the smallest particle of an element that may or may not exist independently, e.g., H, O.

  • A molecule is a group of two or more atoms chemically combined, and it can exist independently, e.g., H₂, H₂O.

Q8. Why do we use brackets while writing formulae of compounds like Ca(OH)₂ and not in NaOH? (2 Marks)

Ans: Brackets are used when more than one polyatomic ion is present.

  • In Ca(OH)₂, there are two hydroxide (OH⁻) ions.

  • In NaOH, only one OH⁻ is present, so brackets are not needed.

Q9. Explain Dalton's atomic theory postulates that support the Law of Conservation of Mass and Law of Definite Proportions. (3 Marks)

Ans:Dalton's atomic theory includes the following postulates that support the two laws:

  1. Atoms are indivisible particles that cannot be created or destroyed in a chemical reaction.
    → This supports the Law of Conservation of Mass, as no atoms are lost or gained during a reaction.

  2. Atoms of an element combine in fixed whole-number ratios to form compounds.
    → This supports the Law of Definite Proportions, since a compound always contains the same elements in the same fixed ratio by mass.

  3. The relative number and kinds of atoms are constant in a given compound, ensuring the same composition every time it forms.

These postulates explain why mass is conserved and composition remains fixed in chemical reactions.

Q10. Calculate the ratio by number of atoms for ammonia (NH3) using atomic masses. (3 Marks)

Ans:
Unit Test (Solutions): Atoms and MoleculesGiven:

  • Atomic mass of Nitrogen (N) = 14 u

  • Atomic mass of Hydrogen (H) = 1 u

  • Ammonia formula = NH₃ (1 nitrogen atom and 3 hydrogen atoms)

Step 1: Calculate the molar mass of ammonia (NH₃):
= (1 × 14 u) + (3 × 1 u)
= 14 u + 3 u
= 17 u

Step 2: Calculate the ratio by number of atoms:

In 1 molecule of NH₃:
→ Nitrogen atoms = 1
→ Hydrogen atoms = 3

So, the ratio by number of atoms (N : H) = 1 : 3


Q11. Write the chemical formulae for: (a) Aluminium oxide, (b) Calcium hydroxide, (c) Sodium sulphate. (3 Marks)

Ans: To write chemical formulae, we use the valency of elements:Unit Test (Solutions): Atoms and MoleculesSo,
(a) Al2O3
(b) Ca(OH)2
(c) Na2SO4

Q12. Explain how to write the chemical formula of a compound using valency, with examples of magnesium chloride and ammonium sulphate. (5 Marks)

Ans: To write the chemical formula of a compound, follow these steps:

Step 1: Write the symbols and valencies of the ions.

We need to know the combining capacity (valency) of each element or ion.

Example 1: Magnesium Chloride

  • Magnesium (Mg) has a valency of 2 → Mg²⁺

  • Chlorine (Cl) has a valency of 1 → Cl⁻

Step 2: Cross the valencies to balance the charges.
Unit Test (Solutions): Atoms and Molecules

Chemical formula:

➡️ MgCl₂

Example 2:Ammonium Sulphate
  • Ammonium ion (NH₄⁺) has a valency of 1
  • Sulphate ion (SO₄²⁻) has a valency of 2
Step 2: Cross the valencies

Unit Test (Solutions): Atoms and MoleculesChemical formula:

➡️ (NH₄)₂SO₄

Q13. Calculate the formula unit mass of: 
(a) NaCl, 
(b) CaCO3,
Also, verify the Law of Conservation of Mass using the reaction: 5.3 g sodium carbonate + 6 g  acetic acid → 8.2 g sodium acetate + 2.2 g carbon dioxide + 0.9 g water (5 Marks)

Ans:

(a) Sodium Chloride (NaCl):

  • Sodium (Na) = 23 u

  • Chlorine (Cl) = 35.5 u

Formula unit mass of NaCl = 23 + 35.5 = 58.5 u

(b) Calcium Carbonate (CaCO₃):

  • Calcium (Ca) = 40 u

  • Carbon (C) = 12 u

  • Oxygen (O) = 16 u × 3 = 48 u

Formula unit mass of CaCO₃ = 40 + 12 + 48 = 100 u


Verification of Law of Conservation of Mass:
Reactants: Sodium carbonate (5.3 g) + Acetic acid (6 g) = 11.3 g.
Products: Sodium acetate (8.2 g) + Carbon dioxide (2.2 g) + Water (0.9 g) = 8.2 + 2.2 + 0.9 = 11.3 g.
Since total mass of reactants = total mass of products,
Law of Conservation of Mass is verified.
→ Mass is neither created nor destroyed in a chemical reaction.

The document Unit Test (Solutions): Atoms and Molecules is a part of the Class 9 Course Science Class 9.
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FAQs on Unit Test (Solutions): Atoms and Molecules

1. What are atoms and molecules, and how do they differ?
Ans. Atoms are the basic building blocks of matter, consisting of a nucleus surrounded by electrons. A molecule, on the other hand, is formed when two or more atoms bond together. The key difference is that atoms represent a single unit of an element, while molecules can consist of the same or different types of atoms bonded together.
2. How are chemical formulas used to represent molecules?
Ans. Chemical formulas use symbols from the periodic table to represent the elements in a molecule and subscripts to indicate the number of each type of atom present. For example, the formula H₂O indicates that each water molecule consists of two hydrogen atoms and one oxygen atom.
3. What is the significance of the law of conservation of mass in chemical reactions?
Ans. The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction. This principle is significant because it means that the total mass of the reactants must equal the total mass of the products, allowing chemists to predict the outcomes of reactions and balance chemical equations accurately.
4. Can you explain the concept of atomic mass and its importance?
Ans. Atomic mass is the weighted average mass of an atom of an element, measured in atomic mass units (amu). It is important because it helps in understanding the relative abundance of isotopes and is essential for calculating the molar mass of compounds, which is crucial for stoichiometry in chemical reactions.
5. What role do ions play in the formation of compounds?
Ans. Ions are charged particles that form when atoms lose or gain electrons. Positive ions (cations) and negative ions (anions) attract each other to form ionic compounds. This interaction is fundamental in the formation of various compounds, such as table salt (NaCl), where Na⁺ and Cl⁻ ions combine to create a stable structure.
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Apr 15, 2026 Last updated
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