IMO Practice Test: Atoms and Molecules | Class 9 Chemistry

Question 1 of 14

The number of oxygen atoms in 0.5 mole of CO₂ is:

6.022×10²³

3.011×10²³

1.2044×10²⁴

6.022×10²²

Explanation: 0.5 mole CO₂ has 0.5 × 6.022×10²³ molecules; each has 2 O atoms, so O atoms = 1 × 6.022×10²³ = 6.022×10²³.

Question 2 of 14

Which sample contains the largest number of atoms?

1 g of hydrogen (H₂)

1 g of oxygen (O₂)

1 g of carbon (C)

1 g of sodium (Na)

Explanation: Atoms ∝ (mass/atomic mass) × atomicity. H₂: (1/2)×2 = 1 mole of atoms — the most; the others give far fewer moles of atoms.

Question 3 of 14

An element X forms an oxide X₂O₃. The valency of X is:

1

2

3

4

Explanation: Oxygen has valency 2; criss-cross of X₂O₃ shows X has valency 3.

Question 4 of 14

The mass of 6.022×10²² molecules of CO₂ (molar mass 44 g) is:

4.4 g

44 g

0.44 g

8.8 g

Explanation: Moles = 6.022×10²² ÷ 6.022×10²³ = 0.1; mass = 0.1 × 44 = 4.4 g.

Question 5 of 14

Two oxides of a metal contain 50% and 60% oxygen by mass. The data best follow the law of:

conservation of mass

constant proportions

multiple proportions

definite volumes

Explanation: When two compounds of the same elements have different fixed ratios, it illustrates the law of multiple proportions.

Question 6 of 14

The number of moles of water molecules in 1.8 g of water (molar mass 18 g) is:

1

0.1

0.01

10

Explanation: n = 1.8 ÷ 18 = 0.1 mole.

Question 7 of 14

Assertion: One mole of N₂ and one mole of CO₂ contain the same number of molecules. Reason: One mole of any substance contains the Avogadro number of particles.

Both true, reason explains assertion

Both true, reason does not explain assertion

Assertion true, reason false

Assertion false, reason true

Explanation: Both are true and the reason (every mole has 6.022×10²³ particles) correctly explains the assertion.

Question 8 of 14

The total number of atoms in 1 mole of CaCO₃ is:

5 × 6.022×10²³

3 × 6.022×10²³

6.022×10²³

100 × 6.022×10²³

Explanation: CaCO₃ has 1 + 1 + 3 = 5 atoms per formula unit, so 1 mole has 5 × 6.022×10²³ atoms.

Question 9 of 14

Equal masses of CO₂ and O₂ are taken. The ratio of their number of moles (CO₂ : O₂) is:

1 : 1

8 : 11

11 : 8

44 : 32

Explanation: For equal mass m, moles = m/44 and m/32; ratio = (1/44):(1/32) = 32 : 44 = 8 : 11.

Question 10 of 14

The percentage of nitrogen by mass in ammonium nitrate NH₄NO₃ (molar mass 80 g) is:

17.5%

35%

28%

14%

Explanation: Mass of N = 2 × 14 = 28 g; % N = (28/80) × 100 = 35%.

Question 11 of 14

A sample of 8 g of an element contains 0.25 mole of atoms. Its atomic mass is:

16 u

32 u

8 u

64 u

Explanation: Atomic mass = mass ÷ moles = 8 ÷ 0.25 = 32 u.

Question 12 of 14

The number of hydrogen atoms in 17 g of ammonia NH₃ (molar mass 17 g) is:

6.022×10²³

1.8066×10²⁴

3.011×10²³

1.2044×10²⁴

Explanation: 17 g NH₃ = 1 mole = 6.022×10²³ molecules; each has 3 H atoms, so H atoms = 3 × 6.022×10²³ = 1.8066×10²⁴.

Question 13 of 14

Which of the following has the greatest mass?

1 mole of O₂

0.5 mole of CO₂

2 moles of H₂

0.5 mole of N₂

Explanation: Masses: O₂ = 32 g, 0.5 CO₂ = 22 g, 2 H₂ = 4 g, 0.5 N₂ = 14 g. The largest is 1 mole of O₂ = 32 g.

Question 14 of 14

If 3 g of carbon combines with oxygen to give 11 g of CO₂, the mass of oxygen that combined is:

8 g

11 g

14 g

3 g

Explanation: By conservation of mass, oxygen = 11 − 3 = 8 g (and C : O = 3 : 8, as expected).

IMO Practice Test — Atoms and Molecules