Home / iGCSE Chemistry (0620) Core:3.2 Relative masses of atoms and molecules: Exam Style Questions Paper 1

iGCSE Chemistry (0620) Core:3.2 Relative masses of atoms and molecules: Exam Style Questions Paper 1

iGCSE Chemistry (0620) Core Paper 1 - All Chapters
Question

What is the relative formula mass of magnesium bromide?

A) 47
B) 82
C) 104
D) 184

▶️ Answer/Explanation
Solution

Ans: D

First, we need to know:

  • Magnesium (Mg) has atomic mass ≈ 24
  • Bromine (Br) has atomic mass ≈ 80

The formula for magnesium bromide is MgBr₂ (magnesium has 2+ charge, bromide has 1- charge).

Calculating relative formula mass:

\[ \text{MgBr}_2 = 24 + (2 \times 80) = 24 + 160 = 184 \]

Therefore, the correct answer is 184.

Common mistake would be forgetting there are two bromine atoms in the formula.

Question

What is the relative molecular mass, Mr, of sulfuric acid, H2SO4?

A) 81
B) 82
C) 97
D) 98

▶️ Answer/Explanation
Solution

Ans: D

To calculate the relative molecular mass (Mr) of sulfuric acid (H2SO4):

  1. Hydrogen (H): 2 atoms × 1 = 2
  2. Sulfur (S): 1 atom × 32 = 32
  3. Oxygen (O): 4 atoms × 16 = 64

Total Mr = 2 + 32 + 64 = 98

Remember to use the relative atomic masses from the periodic table:

  • H = 1
  • S = 32
  • O = 16

The correct answer is therefore D) 98.

Question

 The relative atomic mass of chlorine is 35.5 .
When calculating relative atomic mass, which particle is the mass of a chlorine atom compared to?

A a neutron

B a proton

C an atom of carbon-12

D an atom of hydrogen-1

▶️Answer/Explanation

Ans:C

When calculating the relative atomic mass, the mass of a chlorine atom is compared to the mass of a carbon-12 atom. The relative atomic mass of an element is determined by comparing the average mass of its atoms to the mass of a carbon-12 atom, which is defined as exactly 12 atomic mass units (amu).

Question

What is the relative formula mass of magnesium nitrate, \(Mg(NO_3)_2\)?
A 74           B 86           C 134            D 148

▶️Answer/Explanation

Ans: D

The relative formula mass of a molecule is the sum of the atomic masses of all the atoms in its chemical formula. It is also known as the molecular mass or molecular weight.

The relative formula mass of magnesium nitrate, \(Mg(NO_3)_2\) = 24 + 2*14+6*16 = 148.

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