Mastering Eudiometry: Gaseous Reactions Made Simple

Eudiometry is the analysis of gas mixtures by measuring volume changes during chemical reactions. A gas sample is exploded or reacted in a eudiometer — a graduated tube over mercury or water — and the shrinkage in gas volume reveals the mixture's composition. It is stoichiometry done with burette readings instead of a weighing balance.

Eudiometry matters because it was historically how chemists determined the composition of air and fuel gases, and it remains a beautiful exam topic that fuses gas laws with mole ratios. The entire method rests on two laws: Gay-Lussac's law of combining volumes and Avogadro's law. Master it and you prove you truly understand that equal volumes of gases mean equal moles.

Key Concepts

• Eudiometer: a graduated gas-measuring tube in which the reaction (often sparked explosion) is carried out and volume changes read directly.
• Gay-Lussac's law of combining volumes: when gases react, their volumes are in small whole-number ratios — 2 volumes of hydrogen + 1 volume of oxygen → water.
• Avogadro's law: equal volumes of gases at the same temperature and pressure contain equal numbers of molecules, so volume ratios equal mole ratios.
• Contraction: the decrease in total gas volume after reaction: contraction = initial gas volume − final gas volume.
• Explosion method: a spark ignites a hydrogen/oxygen or hydrocarbon/oxygen mixture; water formed condenses to liquid and vanishes from the gas volume.
• Absorption methods: specific gases are removed selectively — KOH absorbs CO2, alkaline pyrogallol absorbs O2 — and each volume drop measures that component.

Formulas

At the same temperature and pressure:

Volume ratio of reacting gases = mole ratio from the balanced equation

Contraction = (total initial gas volume) − (total final gas volume)

Limits: only gases count in the volume ratios — liquids and solids (like condensed water) contribute zero gas volume. Temperature and pressure must stay constant between readings, and the gases must behave ideally enough that Avogadro's law holds.

Worked Example

30 mL of hydrogen is exploded with 20 mL of oxygen in a eudiometer at room temperature. Find the contraction and the composition of the residual gas. Reaction: 2H2 + O2 → 2H2O(l).

1. Volume ratio H2:O2 = 2:1, so 30 mL H2 needs 30 ÷ 2 = 15 mL O2.
2. Oxygen supplied is 20 mL, so oxygen is in excess and hydrogen limits.
3. Oxygen consumed = 15 mL; oxygen left = 20 − 15 = 5 mL.
4. Water formed is liquid at room temperature → contributes 0 mL of gas.
5. Final gas volume = 5 mL (leftover oxygen only).
6. Contraction = (30 + 20) − 5 = 45 mL. The residual gas is 5 mL of oxygen.

Common Misconceptions

• "The water formed counts in the final gas volume." At room temperature the product water condenses to liquid, which occupies negligible volume. Forgetting this and counting 2 volumes of "water gas" is the most common eudiometry error.
• "Volume ratios work for solids and liquids too." Gay-Lussac's law applies only to gases. The volumes of liquid water or solid carbon in the equation never enter the ratio.
• "You can compare volumes measured at different temperatures." Gas volumes change with temperature and pressure, so initial and final readings must be taken under the same conditions — otherwise cool everything back down first.

Real-World Applications

• Fuel-gas analysis: coal gas and natural gas were historically analysed by explosion eudiometry to determine their heating value.
• Air composition: early chemists measured oxygen in air by absorbing it with pyrogallol and reading the volume drop — eudiometry in action.
• Mine safety: detecting explosive methane-air mixtures relies on the same volume-ratio combustion principles.
• Teaching laboratories: eudiometers still demonstrate Gay-Lussac's and Avogadro's laws more vividly than any textbook diagram.

Eudiometry is stoichiometry with a measuring tube: volume ratios mirror mole ratios, condensed products drop out of the gas count, and contraction tells you how much reacted. Work every problem by writing the equation, converting volumes with the mole ratio, and subtracting carefully — do that, and gas-analysis questions become some of the most reliable marks on the paper.