10 ml of a gaseous hydrocarbon is mixed with excess of oxygen and burnt. The gases are then cooled to laboratory temperature. The reduction in volume was found to be 25 ml. When the gases are passed into caustic potash, there is a further reduction in volume of 30 ml. All volumes are measured under the laboratory conditions. The formula of the hydrocarbon is-

Let the formula of the hydrocarbon be CxHy. It is mixed with excess oxygen and burnt i.e. combustion is taking place which can be shown as follows:
CxHy + (x + y4)O2      xCO2 + y2 H2O1 vol.      (x+y4)mL          x vol.10 mL    10(x+y4)mL       10x mL
The gases are allowed to cool and vol. of gaseous mixture is recorded. 
The volume corresponds to unreacted oxygen and carbon dioxide.
The volume of carbon dioxide is determined by decrease in the volume of the gaseous mixture when the mixture is passed into caustic potash.
 
The first reduction in volume = 10 + 10(x+y/4)-10x = 25
                                              = 10+ 10y/4= 25
                   Thus, y= 6
Volume of carbon dioxide produced= 30 mL
Thus, 10x = 30
            x= 3

Hence, the molecular formulaof the hydrocarbon = C3H6


 

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