This worked example covers Worked Example — Full Method within Moles & Calculations for GCSE Chemistry. Revise Moles & Calculations in Quantitative Chemistry for GCSE Chemistry with 27 exam-style questions and 20 flashcards. This topic shows up very often in GCSE exams, so students should be able to explain it clearly, not just recognise the term. It is section 9 of 17 in this topic. Treat this as a marking guide for what examiners are looking for, not just a fact list.
🧮 Worked Example — Full Method
Question: What mass of magnesium oxide forms when 4.8g of magnesium burns completely?
Equation: 2Mg + O₂ → 2MgO (Ar: Mg = 24, O = 16)
2Mg + O₂ → 2MgO ✓
Known: 4.8g of Mg
Unknown: mass of MgO
n = m ÷ Mr
n = 4.8 ÷ 24
n = 0.2 mol of Mg
From equation: 2Mg : 2MgO = 1:1 ratio
So 0.2 mol Mg produces 0.2 mol MgO
Mr of MgO = 24 + 16 = 40
m = n × Mr
m = 0.2 × 40
m = 8.0g
Started with 4.8g Mg, added oxygen → got 8.0g product
Mass increased (oxygen added) ✓ Makes sense!
Answer: 8.0g of magnesium oxide
Practice questions for Moles & Calculations
One mole of any substance contains how many particles?
Explain why the percentage yield of a reaction is never 100% in practice.