In physical chemistry, Eudiometry (also known as Gas Analysis) is a technique used to determine the exact molecular formula of an unknown gas mixture, specifically hydrocarbons. It relies entirely on Avogadro's Law, which states that equal volumes of all gases contain equal numbers of molecules (under the same temperature and pressure).
Eudiometry problems are almost always solved under Standard Temperature and Pressure (STP).
The name comes from the Greek words 'eudios' (clear weather) and 'metron' (measure). It was originally invented to test the purity of the air.
A Eudiometer is simply a long, heavy glass tube closed at one end, equipped with two platinum wires. Scientists inject a mixture of an unknown gas and Oxygen into the tube, and then fire an electrical spark through the wires to cause an explosion (combustion). By measuring how much the gas volume shrinks after the explosion, they can calculate the formula of the gas.
The entire math of Eudiometry is based on balancing the chemical equation for the combustion of a generic hydrocarbon ($C_xH_y$). If you memorize this one balanced formula, you can solve any eudiometry problem.
Equation: $C_xH_y + (x + \frac{y}{4})O_2 \longrightarrow xCO_2 + (\frac{y}{2})H_2O(liquid)$
Because of Avogadro's Law, the stoichiometric coefficients in the equation directly represent the Volume of the gases.
(Crucial Note: The water ($H_2O$) produced is assumed to be a liquid at room temperature, so its volume is considered zero in gas calculations).
Volume Contraction (Shrinkage): After the explosion, the total volume of gases shrinks. The formula for this contraction is: Contraction = (Initial Volume of Gases) - (Final Volume of Gases) Contraction = $V [1 + \frac{y}{4}]$
After the explosion, scientists must figure out what gases are left in the tube. They do this by passing the remaining gas through specific chemical absorbers:
During combustion in a eudiometer, Nitrogen gas ($N_2$) is extremely unreactive. It does not burn. It simply remains in the tube as $N_2$ gas and its volume is measured at the very end.
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