ChemistryChapter 12: Analytical Chemistry
Combustion Analysis
Classical combustion analysis finds empirical (and with molar mass, molecular) formulas for compounds containing only C and H, or C, H, and
Learning objectives
- Explain combustion analysis in clear and easy chemistry language.
- Use the important definitions, equations, examples, and conditions correctly.
- Apply the idea in an FBISE-style question and avoid the common exam mistake.
Definition
Classical combustion analysis finds empirical (and with molar mass, molecular) formulas for compounds containing only C and H, or C, H, and O. Weigh the sample, burn completely to CO2 and H2O, and trap products in weighed absorber tubes: one absorbs H2O, one absorbs CO2. Mass gain of water tube → mass of H; mass gain of CO2 tube → mass of C. If oxygen is also present, mass of O = sample mass − mass C − mass H. Convert masses to moles, find simplest ratio (empirical formula), then use molecular mass to get molecular formula (n = M / Mempirical).
Mathematical form
O = sample mass
n = M / Mempirical). Combustion
Important terms
- COMBUSTION ANALYSIS
- Classical combustion analysis finds empirical (and with molar mass, molecular) formulas for compounds containing only C and H, or C, H, and O.
- EXPERIMENTAL STEPS
- Complete combustion in oxygen stream; dry CO2/H2O drawn through absorbers; weigh before/after.
- TO FORMULAS
- Mass C from CO2; mass H from H2O; moles → empirical; molecular = (empirical)n.
Overview
Apparatus idea, then calculation path:
Experimental Steps
Complete combustion in oxygen stream; dry CO2/H2O drawn through absorbers; weigh before/after. Only C/H/(O) compounds are suited to this classical method.
Examples Water absorber first · CO2 absorber second · Hydrocarbon → CO2 + H2O only · Oxygenated compound needs O by difference
To Formulas
Mass C from CO2; mass H from H2O; moles → empirical; molecular = (empirical)n. Trioxane worked example in the book shows the full arithmetic.
Examples g C from g CO2 · g H from g H2O · CH2O empirical style · C3H6O3 molecular if n=3
Core idea — COMBUSTION ANALYSIS
Classical combustion analysis finds empirical (and with molar mass, molecular) formulas for compounds containing only C and H, or C, H, and O. Weigh the sample, burn completely to CO2 and H2O, and trap products in weighed absorber tubes: one absorbs H2O, one absorbs CO2. Mass gain of water tube → mass of H; mass gain of CO2 tube → mass of C. If oxygen is also present, mass of O = sample mass − mass C − mass H. Convert masses to moles, find simplest ratio (empirical formula), then use molecular mass to get molecular formula (n = M / Mempirical).
1. Experimental Steps
Complete combustion in oxygen stream; dry CO2/H2O drawn through absorbers; weigh before/after. Only C/H/(O) compounds are suited to this classical method. Examples: Water absorber first; CO2 absorber second; Hydrocarbon → CO2 + H2O only; Oxygenated compound needs O by difference.
2. To Formulas
Mass C from CO2; mass H from H2O; moles → empirical; molecular = (empirical)n. Trioxane worked example in the book shows the full arithmetic. Examples: g C from g CO2; g H from g H2O; CH2O empirical style; C3H6O3 molecular if n=3.
Worked method and exam use
Combustion: CO2/H2O absorbers → %C/%H (/O) → empirical → molecular with M.
Explanation
Combustion: CO2/H2O absorbers → %C/%H (/O) → empirical → molecular with M.
In simple words
Drawback: combustion analysis cannot find all element types beyond C/H/O — modern methods needed.
Key points
- burn → CO2 + H2O
- absorber mass gains
- O by difference
- weigh · burn · absorb · reweigh
- limitation to CHO
Exam preparation
Exam Focus
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.Combustion Analysis
Sticky Notes · Quick Revision
Core idea
- Classical combustion analysis finds empirical (and with molar mass, molecular) formulas for compounds containing only C and H, or C, H, and O. Weigh the sample, burn completely to CO2 and H2O, and trap products in weighed absorber tubes: one absorbs H2O, one absorbs CO2. Mass gain of water tube → mass of H; mass gain of CO2 tube → mass of C. If oxygen is also present, mass of O = sample mass − mass C − mass H. Convert masses to moles, find simplest ratio (empirical formula), then use molecular mass to get molecular formula (n = M / Mempirical).
Experimental Steps
- Complete combustion in oxygen stream; dry CO2/H2O drawn through absorbers; weigh before/after. Only C/H/(O) compounds are suited to this classical method.
To Formulas
- Mass C from CO2; mass H from H2O; moles → empirical; molecular = (empirical)n. Trioxane worked example in the book shows the full arithmetic.
Worked method
- Combustion: CO2/H2O absorbers → %C/%H (/O) → empirical → molecular with M.
Common misconception
- Drawback: combustion analysis cannot find all element types beyond C/H/O — modern methods needed.
FBISE exam check
- In an exam answer, include these key points: burn → CO2 + H2O; absorber mass gains.
- Check: CHO limitation.
60-second revision
- Classical combustion analysis finds empirical (and with molar mass, molecular) formulas for compounds containing only C and H, or C, H, and O.
- Experimental Steps: Complete combustion in oxygen stream; dry CO2/H2O drawn through absorbers; weigh before/after.
- To Formulas: Mass C from CO2; mass H from H2O; moles → empirical; molecular = (empirical)n.
- Exam reminder: Drawback: combustion analysis cannot find all element types beyond C/H/O — modern methods needed.
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