The percentage by mass of Oxygen in MgSO$_4$.7H$_2$O is [ M$_r$ of MgSO$_4$.7H$_2$O = 246 ]

Analyze the question: We need the percentage by mass of oxygen in hydrated magnesium sulfate, MgSO₄·7H₂O, with Mr = 246.

Concept: Percentage of an element = (total mass of that element in one formula unit ÷ molar mass of the compound) × 100

Solution:

Count the oxygen atoms:

• In MgSO₄: 4 oxygen atoms
• In 7H₂O: 7 oxygen atoms
• Total = 11 oxygen atoms

Mass of oxygen = 11 × 16 = 176

Percentage of oxygen = (176 ÷ 246) × 100 = 71.5%

Check the Mr: Mg (24) + S (32) + 4O (64) = 120. 7H₂O = 7 × 18 = 126. Total = 246 ✔

Answer: The percentage of oxygen is 71.5%, which is option C.

Common mistakes:

• Option B (45.5%) is not what you get if you forget the water: counting only 4 oxygens gives 64/246 = 26.0% (option A, a trap).
• Option D (84.0%) is not obtained by any correct approach here.

Tip: For hydrated salts, always count the oxygen from the water of crystallization too. Write the total number of atoms first (here 4 + 7 = 11) before calculating, so you do not miss any.