Q: What does H2 Chemistry Notes: CORE IDEA 3, Topic 6 - Mole Concept and Stoichiometry cover? A: Build systematic problem-solving routines for mole calculations, limiting reagents, redox titrations, and analytical stoichiometry in Core Idea 3 (Mole Concept and Stoichiometry).
Stoichiometry underpins quantitative chemistry-from gas calculations to titration analysis. This note structures the workflow and highlights the must-know techniques for Paper 2 and Paper 3.
Status: SEAB H2 Chemistry (9729, first exam 2026) syllabus and Chemistry Data Booklet last checked 2025-11-29 (syllabus PDF last modified 2024-07-17; booklet last modified 2024-12-17). Core Idea 3 Topic 6 expectations remain unchanged.
1 Fundamental Relationships
Formula
Description
n=Mm
Moles from mass and molar mass.
n=CV
Moles from concentration C (mol⋅L−1) and volume V (L).
n=VmV
Moles from gas volume V and molar volume Vm=24.0dm3⋅mol−1 at 298K and 1.01⋅105Pa.
Always state units. If conditions differ from RTP, revert to the ideal gas equation PV=nRT. The molar gas volume above matches the SEAB Chemistry Data Booklet (exams from 2026) conditions; use any alternative value provided in the question stem.
2 Stoichiometric Method
Write a balanced equation.
Convert all given quantities to moles.
Use mole ratios from the equation to relate substances.
Convert back to required quantity (mass, volume, concentration).
When numerical work is required, take relative atomic masses and constants directly from the SEAB Chemistry Data Booklet (exams from 2026) rather than rounded memory values.
2.1 Limiting Reagent Logic
Calculate moles of each reactant and compare the ratio with the balanced equation. The reactant yielding the smallest amount of product is limiting. Show working to secure method marks.
Percentage purity:%purity=total mass of samplemass of pure substance×100
Use mass or moles consistently throughout. For purity problems, the impure mass is often the quantity measured experimentally; set up stoichiometric equations using only the pure component.
4 Redox and Acid-Base Titrations
4.1 Typical Workflow
Write ionic equations (especially for redox).
Convert primary standard volume x concentration into moles.
Apply mole ratio to find moles of analyte.
Convert to requested quantity (concentration, mass, % purity).
4.2 Common Redox Equations
MnOX4X− in acidic medium:
MnOX4X−+8HX++5eX−MnX2++4HX2O
CrX2OX7X2−
SX2OX3X2−
State oxidation numbers to justify electron counts if required.
5 Empirical and Molecular Formulae
Divide percentage or mass data by relative atomic mass to get mole ratio.
Divide all moles by the smallest value to obtain simplest whole-number ratio.
Determine molecular formula using molar mass:
MempiricalMmolecular=integer multiplier
Be ready for combustion analysis questions: convert mass of COX2 and HX2O to moles to deduce carbon and hydrogen content.
6 Worked Example
Question:
An impure sample of potassium iodide (KI) weighing 0.700g is titrated with 0.0200molL−1KX2CrX2OX7 in acidic solution. 24.80mL of dichromate is required to reach the endpoint. Determine the percentage purity of the KI sample.
Moles of dichromate:n=CV=(0.0200molL−1)(2.480⋅10−2L)=4.96⋅10−4mol
Moles of iodide: using the 1:6 ratio → n(IX−)=6×4.96⋅10−4mol=2.98⋅10−3mol
Mass of pure KI:m=nM=2.98⋅10−3mol×166.0gmol−1=0.495g
Percentage purity:0.700g0.495g×100=70.7%.
Statement: The KI sample is 70.7% pure.
Remember to report with appropriate significant figures based on experimental data.
7 Practical Tips
Use consistent decimal places in titration tables (e.g. two decimal places for burette readings).
In Paper 4 planning sections, specify standard solutions (e.g. primary standard NaX2COX3 for acid standardisation) and justify choice (stable, high purity).
Mention safety considerations for oxidising agents (KMnOX4, KX2CrX2OX7
8 Common Mistakes
Forgetting dilution effect after mixing solutions.
Applying molar ratios incorrectly when coefficients differ.
Ignoring spectator ions in ionic equations, leading to unbalanced charge.
Using molar volume 24.0dm3⋅mol−1 at non-RTP conditions.
9 Quick Drills
A hydrate CuSOX4⋅xHX2O loses mass from 5.00g to 3.20g upon heating. Determine x.
Calculate the mass of CaCOX3 required to neutralise 25.0mL of 2.00mol⋅L−1
A gaseous hydrocarbon combusts to produce 2.64g of COX2 and 1.08g of HX2O
Check answers with method sheets to ensure your working lines follow the balanced-equation → mole ratio → final quantity structure.
Fluent stoichiometry keeps later topics (equilibria, kinetics, redox) manageable. Keep rehearsing with mixed-problem sets and consult https://eclatinstitute.sg/blog/h2-chemistry-notes for integrated practice.
H2 Chemistry Notes: CORE IDEA 3, Topic 6 - Mole Concept and Stoichiometry
