Multi-Step Mole Calculations
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Lesson overview
Combining mass, molar mass and concentration in one problem.
Go to lesson quiz →Exam guidance · WAEC WASSCE Paper 1 + Paper 2 Lens
Core idea (Mole Concept, Molar Mass & Stoichiometry): Convert every mass, gas volume, or concentration to moles first, apply the balanced mole ratio, then convert to the target unit.
Pacing
Objective speed on Paper 1 + structured step-by-step method marks on Paper 2 Theory/Practical.
How WAEC marks this
Examiners award method marks (M1) for stating the formula/definition and accuracy marks (A1) ONLY when SI units are written.
Watch out for
Writing bare numbers without working or omitting SI units loses up to 50% of Paper 2 marks even when the final value is right.
Lesson content
Overview — Multi-Step Mole Calculations
Combining mass, molar mass and concentration in one problem. This lesson is at the advanced level — by the end, you'll be able to recognise this question type, apply a reliable method, and avoid the traps that cost marks.
Key Concept
Complex mole problems often chain several formulas together — for example, finding moles from mass, then using those moles to find a concentration or a mass of a different substance.
Why It's Tested This Way
Chaining mole calculations together (mass → moles → concentration, or vice versa) is exactly how WAEC/JAMB separate top-band Advanced answers from Intermediate ones — it tests whether the mole concept has become second nature.
Method
The key strategy is to work step by step: identify what you're given, what you need to find, and which mole formula connects them, solving one step at a time.
Worked Example
Worked example: To find the concentration of a solution made by dissolving 20 g of NaOH (molar mass 40 g/mol) in 2 dm³ of water: moles = 20 ÷ 40 = 0.5 mol, then concentration = 0.5 ÷ 2 = 0.25 mol/dm³.
Second Worked Example
Second worked example: To find the mass of solute in 500 cm³ (0.5 dm³) of a 2 mol/dm³ solution of NaOH: moles = 2 × 0.5 = 1 mol, then mass = 1 × 40 = 40 g.
Common Traps
Common trap: trying to skip a step and combine formulas incorrectly — it's safer to always calculate moles first as an intermediate step.
Lesson Summary
Remember: (1) identify what is being asked, (2) apply the WAEC strategy, (3) check against the traps. Complete the 10-question quiz below to unlock the next lesson — you need at least 60% to pass.
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