Key Learning Objectives
- Calculate amount of substance in moles using Moles = Mass ÷ Molar Mass (Mr).
- Determine the empirical formula of compounds from experimental mass or percentage composition data.
- Deduce molecular formulas given empirical formulas and relative formula masses.
Topic Summary & Core Notes
Master the mole equation (Moles = Mass ÷ Mr) and solve empirical formula problems systematically using mass tables, mole ratios, and simplest whole-number conversions.
Key Rules, Formulas & Definitions
Mole Formula
Moles (n) = Mass (m in grams) ÷ Relative Formula Mass (Mr) [n = m / Mr]
Empirical Ratio
Ratio = Moles of each element ÷ Smallest mole value
2 Step-by-Step Worked Examples
Worked Example 1
Worked Example 1: Empirical Formula Calculation
Question: A compound contains 4.8 g of magnesium and 3.2 g of oxygen. Calculate its empirical formula. [Ar: Mg = 24, O = 16]
Step-by-Step Solution:
- Step 1: Calculate moles: Moles of Mg = 4.8 ÷ 24 = 0.20 mol. Moles of O = 3.2 ÷ 16 = 0.20 mol.
- Step 2: Divide by smallest: Mg: 0.20 ÷ 0.20 = 1. O: 0.20 ÷ 0.20 = 1.
- Step 3: Deduce formula: 1:1 ratio → MgO.
Final Answer: MgO
Senior Examiner Insight: Always set up a neat 4-row table: Mass → Moles (÷ Ar) → Ratio (÷ smallest) → Simplest Formula.
Worked Example 2
Worked Example 2: Determining Molecular Formula from Empirical Formula
Question: A hydrocarbon has the empirical formula CH2 and a relative formula mass (Mr) of 56. Find its molecular formula. [Ar: C = 12, H = 1]
Step-by-Step Solution:
- Step 1: Calculate empirical unit mass: Empirical mass of CH2 = 12 + (2 × 1) = 14.
- Step 2: Find scaling multiplier: Multiplier = Molecular Mr ÷ Empirical mass = 56 ÷ 14 = 4.
- Step 3: Multiply empirical subscripts: C(1×4) H(2×4) = C4H8.
Final Answer: C4H8
Senior Examiner Insight: Show the multiplier division (56 ÷ 14 = 4) to secure the method mark.
Common Pitfalls & Examiner Warnings
Common Mistake: Dividing masses by atomic number (Z) instead of atomic mass (Ar)
Always use the larger mass number from the Periodic Table when calculating moles.
Common Mistake: Rounding non-integer ratios prematurely
A ratio of 1.5 indicates 3:2 (multiply by 2), NOT 1:1 or 2:1.
Interactive Self-Check Quiz (3 Questions)
Question 1 of 3
How many moles are present in 44 g of carbon dioxide (CO2)? [Ar: C = 12, O = 16]
Click to Reveal Answer & Mark Scheme
Correct Answer: B) 1.0 mol
Detailed Explanation: Mr of CO2 = 12 + (16 × 2) = 44. Moles = 44 g ÷ 44 = 1.0 mol.
Mark Scheme & Scoring Points: 1 mark for 1.0 mol.
Question 2 of 3
If an element ratio works out to 1 : 1.33, what whole-number ratio does this represent?
Click to Reveal Answer & Mark Scheme
Correct Answer: C) 3 : 4
Detailed Explanation: 1.33 is 4/3. Multiplying both terms by 3 gives 3 : 4.
Mark Scheme & Scoring Points: 1 mark for 3 : 4.
Question 3 of 3
What is the empirical formula of glucose, C6H12O6?
Click to Reveal Answer & Mark Scheme
Correct Answer: B) CH2O
Detailed Explanation: Dividing each subscript by the greatest common divisor (6) gives CH2O.
Mark Scheme & Scoring Points: 1 mark for CH2O.
Created by Fiaraz Iqbal
Former Headteacher & Senior Science / Maths Examiner
Fiaraz produces structured video walkthroughs, Tier 3 literacy packs, and exam mark scheme breakdowns to help secondary students achieve top grades.
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