Online Test: Atoms and Molecules | Class 9 Chemistry

Question 1 of 18

The law of conservation of mass was given by:

Dalton

Lavoisier

Proust

Avogadro

Explanation: Antoine Lavoisier stated that mass is neither created nor destroyed in a chemical reaction.

Question 2 of 18

In water, hydrogen and oxygen combine in the mass ratio:

8 : 1

1 : 8

1 : 16

2 : 1

Explanation: Water always has H : O = 1 : 8 by mass, illustrating the law of constant proportions.

Question 3 of 18

Atoms of the same element, according to Dalton, are:

different in mass

identical in all respects

always charged

always divisible

Explanation: Dalton proposed that all atoms of a given element are identical in mass and properties.

Question 4 of 18

One atomic mass unit (u) is one-twelfth the mass of an atom of:

hydrogen

oxygen-16

carbon-12

helium

Explanation: 1 u is defined as exactly one-twelfth of the mass of a carbon-12 atom.

Question 5 of 18

The symbol of iron is:

Ir

In

Fe

I

Explanation: Iron's symbol Fe comes from its Latin name ferrum.

Question 6 of 18

A positively charged ion is called a:

anion

cation

molecule

isotope

Explanation: A cation is formed when an atom loses electrons and becomes positively charged.

Question 7 of 18

The valency of the carbonate ion CO₃²⁻ is:

1

2

3

4

Explanation: The carbonate ion carries a 2− charge, so its valency is 2.

Question 8 of 18

The correct formula of aluminium oxide is:

AlO

Al₂O₃

Al₃O₂

AlO₃

Explanation: Al (valency 3) and O (valency 2) criss-cross to give Al₂O₃.

Question 9 of 18

The atomicity of a sulphur molecule (S₈) is:

2

4

6

8

Explanation: Sulphur exists as S₈, so eight atoms make up one molecule — atomicity 8.

Question 10 of 18

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

28 u

44 u

32 u

40 u

Explanation: 12 + 2(16) = 12 + 32 = 44 u.

Question 11 of 18

The number of particles in one mole is:

6.022×10²³

3.011×10²³

6.022×10²²

1.2×10²⁴

Explanation: One mole always contains the Avogadro number, 6.022×10²³ particles.

Question 12 of 18

The number of moles in 90 g of water (molar mass 18 g) is:

2

5

9

0.2

Explanation: n = 90 ÷ 18 = 5 moles.

Question 13 of 18

The mass of 2 moles of carbon atoms (C = 12) is:

12 g

24 g

6 g

48 g

Explanation: Mass = moles × molar mass = 2 × 12 = 24 g.

Question 14 of 18

The formula unit mass of sodium chloride NaCl (Na = 23, Cl = 35.5) is:

48.5 u

58.5 u

68.5 u

35.5 u

Explanation: 23 + 35.5 = 58.5 u.

Question 15 of 18

The number of molecules in 0.5 mole of any gas is:

6.022×10²³

3.011×10²³

1.2×10²⁴

6.022×10²²

Explanation: 0.5 × 6.022×10²³ = 3.011×10²³ molecules.

Question 16 of 18

The correct formula of calcium phosphate is:

CaPO₄

Ca₂PO₄

Ca₃(PO₄)₂

Ca(PO₄)₂

Explanation: Ca²⁺ and PO₄³⁻ criss-cross to give Ca₃(PO₄)₂.

Question 17 of 18

Which of the following is a polyatomic ion?

Na⁺

Cl⁻

SO₄²⁻

Ca²⁺

Explanation: Sulphate SO₄²⁻ is a group of atoms carrying a net charge — a polyatomic ion.

Question 18 of 18

The number of moles represented by 12.044×10²³ molecules of a gas is:

1

2

0.5

3

Explanation: Moles = 12.044×10²³ ÷ 6.022×10²³ = 2 moles.

Online Test — Atoms and Molecules