Molecular formula of gaseous hydrocarbons (Eudiometry)
Eudiometry is a direct method for calculation of molecular formula of gaseous hydrocarbons without ascertaining the percentage composition of various elements in it and without knowing the molecular weight of the hydrocarbon. The actual method employed includes the subsequent steps,
(a) A known volume of the gaseous hydrocarbon is mixed along with an excess (known or unknown volume) of oxygen in the eudiometer tube maintained in a trough of mercury.
(b) The mixture is exploded through passing an electric spark among the platinum electrodes. The result of it, hydrogen and carbon of the hydrocarbon are oxidised to CO2 and H2O vapours correspondingly.
(c) The tube is specified to cool at room temperature while water vapours condense to give liquid water that has a negligible volume like compared to the volume of water vapours, so, the gaseous mixture left behind in the eudiometer tube later than explosion and cooling contains only CO2 and unused O2.
(d) After that the Caustic potash or caustic soda solution is introduced into the eudiometer tube that absorbs CO2 totally and only unused O2 is left behind.
2NaOH + CO2 → Na2CO3 + H2O
So, the decrease in volume on introducing NaOH or KOH solution provides the volume of CO2 formed. Occasionally, the volume of O2 left unused is found by introducing pyrogallol and noting the reduce in volume.