Combustion analysis problems show up on every AP Chemistry exam and in nearly every general chemistry course — and they trip up students who never quite grasped the underlying logic. You know carbon goes in and CO₂ comes out, but translating product masses into an empirical formula, then into a molecular formula, involves four or five steps where one wrong move cascades into a wrong answer.

This TLDR guide walks you through the entire process, cleanly and concisely. You will learn exactly what the combustion apparatus measures and why, how to convert CO₂ and H₂O masses back into grams of carbon and hydrogen, how to find oxygen (and other elements like nitrogen or sulfur) by difference, and how to reduce mole ratios to whole numbers even when the math gives you an ugly fraction. The final section delivers two full end-to-end worked problems at AP Chemistry exam difficulty, plus a checklist of the mistakes that most often cost students points.

This book is for high school students in AP Chemistry or honors chemistry, college students in General Chemistry I or II, and anyone working through combustion analysis and molecular formula determination for the first time or needing a fast review before an exam. Short by design, it covers what you need and nothing else — no filler, no detours into unrelated organic chemistry.

If your next exam includes a combustion analysis question, read this first.

• Explain what combustion analysis is and how the apparatus traps CO2 and H2O.
• Convert masses of CO2 and H2O into masses and moles of carbon and hydrogen in the original sample.
• Find oxygen (and nitrogen, halogen, sulfur) by mass difference when appropriate.
• Determine an empirical formula from mole ratios and a molecular formula using molar mass.
• Avoid the common arithmetic and conceptual traps that cost students points on exams.

1. 1. What Combustion Analysis Actually Measures
   Introduces the technique, the apparatus, and the key idea that all carbon ends up in CO2 and all hydrogen ends up in H2O.
2. 2. From CO2 and H2O Masses to Grams of C and H
   Walks through the mass-fraction logic that converts product masses back into the masses of carbon and hydrogen in the original sample.
3. 3. Finding Oxygen (and Other Elements) by Difference
   Explains why oxygen cannot be measured directly and how to handle samples containing N, S, or halogens.
4. 4. Empirical Formulas from Mole Ratios
   Shows how to convert masses of each element into moles and reduce the ratio to whole numbers, including handling fractional ratios.
5. 5. From Empirical to Molecular Formula
   Uses molar mass (often from a separate measurement) to find the integer multiplier that turns the empirical formula into the molecular formula.
6. 6. Full Worked Problems and Common Pitfalls
   Two end-to-end problems at exam difficulty plus a checklist of mistakes that tank student scores.