WJEC A-Level Chemistry AS Unit 1, June 2025: Question 7

Chemical Formula of Magnesium Hydrogencarbonate

What this question tests

This question assesses fundamental chemical literacy: identifying the charge on a Group 2 metal cation (Mg²⁺), using given information about a compound polyatomic anion (HCO₃⁻), balancing ionic charges to create a neutral compound, and correctly using brackets in chemical formulae.

Question 7 • 1 Mark (AO1)

Writing the Formula from Constituent Ions

Deducing the formula of an ionic compound containing a polyatomic ion

✅ Correct Answer

Mg(HCO₃)₂

(Brackets around the hydrogencarbonate ion are mandatory; subscripts must be clearly below the baseline)

📐 Step-by-Step Deduction

Identify the cation & charge:
Magnesium is in Group 2 of the Periodic Table, so it loses two electrons to form a Mg²⁺ ion.
Identify the anion & charge:
Given in the question prompt: hydrogencarbonate is HCO₃⁻ (charge = -1).
Balance the overall charge:
Overall compound must be neutral (charge = 0).
(+2) + 2 × (-1) = 0
Therefore, two HCO₃⁻ ions are required for every one Mg²⁺ ion.
Apply brackets to the polyatomic ion:
Enclose HCO₃ in brackets before placing the subscript 2: Mg(HCO₃)₂.

💡 Key Knowledge

Group 2 Valency: All Group 2 metals (Be, Mg, Ca, Sr, Ba) form 2+ ions in ionic salts.
Polyatomic Ions: A radical or molecular ion acts as a single chemical unit.
Bracket Rules: Brackets are required whenever a subscript greater than 1 applies to a polyatomic unit (e.g., OH⁻, NO₃⁻, HCO₃⁻).
Case & Subscripts: Element symbols must have clear capitalization (M capital, g lowercase; H, C, O all uppercase).

🧠 Exam Technique & Examiner Insight

Use provided hints: The examiner explicitly provided the formula of the anion (HCO₃⁻). Always double-check its charge from the question stem before combining.
Write legibly: Examiners cannot award marks if subscripts look like normal numbers on the line (e.g., writing Mg(HCO3)2 instead of clearly lowered subscripts Mg(HCO₃)₂).
Never drop brackets: Omitting brackets changes the meaning entirely, indicating multiple atoms of only the last element rather than the whole group.

❌ Common Errors

MgHCO₃₂ or MgH(CO₃)₂: Missing or misplaced brackets completely invalidates the formula.
MgHCO₃: Assuming a 1:1 ratio by incorrectly thinking magnesium forms an Mg⁺ ion.
MgCO₃: Confusing hydrogencarbonate with carbonate (CO₃²⁻).
mg(HCO₃)₂: Incorrect casing (e.g. writing small 'm' instead of capital 'M' for magnesium).
Including charges in the formula: Writing Mg²⁺(HCO₃⁻)₂—the final formula of a compound must show no charges.

Mark Scheme Breakdown:
• 1 Mark (AO1): Awarded exclusively for the fully correct formula Mg(HCO₃)₂ . No partial credit is available for single-mark formula questions.