10 ml of a gaseous hydrocarbon is mixed with excess of oxygen and burnt The gases are then cooled - Chemistry -

Question

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-

Vartika Jain · Accepted Answer

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