OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2019: Question 5

Determining Oxidation Numbers in Hydrated Salts

What this question tests

This question assesses your ability to apply standard rules for assigning oxidation numbers to elements within complex chemical formulas, specifically dealing with polyatomic ions (nitrites, NO₂⁻) and water of crystallisation in hydrated salts without getting distracted by spectator molecules.

Question 5

Part 5: Finding the Oxidation Number of N

Determining the correct option for Mg(NO₂)₂·3H₂O

✅ Correct Answer: B (+3)

Option B is correct. The oxidation number of nitrogen in the nitrite ion (NO₂⁻) is +3.

Awarded 1 mark for selecting B (AO1.2 - Demonstrating knowledge and understanding of scientific rules/formulae).

💡 Key Knowledge

Oxygen rule: Oxygen almost always has an oxidation number of -2 in compounds (except in peroxides or bonded to fluorine).
Water of crystallisation: Water molecules ( H₂O ) are neutral overall (sum = 0) and can be completely ignored when calculating oxidation states of ions in the salt matrix.
Magnesium rule: Group 2 metals always exhibit a fixed oxidation number of +2 in their compounds.

🧠 Exam Technique

Don't panic when you see complex formulas with dot multipliers like ·3H₂O . Treat the water molecules as neutral entities that contribute 0 to the total charge and isolate the specific ion containing your target element.

❌ Common Errors

Including water: Mistakenly trying to factor the atoms in 3H₂O into the overall algebraic equation for the whole formula unit.
Sign errors: Forgetting the crucial +/- sign before the number. Examiners will reject answers written simply as "3" instead of "+3".
Confusing ions: Confusing the nitrite ion ( NO₂⁻ ) with the nitrate ion ( NO₃⁻ ), which would yield an oxidation number of +5 (Option D).

📐 Step-by-Step Calculation Guide

Isolate the relevant ion: Focus on the nitrite ion, NO₂⁻ , which has an overall charge of -1 .
Set up the equation: Let the oxidation number of Nitrogen be x . Since oxygen is -2 and there are two oxygen atoms:
x + 2(-2) = -1
Solve for x:
x - 4 = -1
x = -1 + 4
x = +3
Confirm with the whole formula: Magnesium is +2 , two NO₂⁻ ions contribute 2 × (-1) = -2 , and 3H₂O contributes 0 . Total charge = +2 + (-2) + 0 = 0 . Perfect!