a mixture of C^₂H^₂ and C₃H₈ including sufficient amount of O₂ occupied a certain volume at 80 mm of hg .the mixture was burnt to CO₂ and H₂O(l).when the pressure of CO₂ was found to be 42.5 mm of hg at the same temp and volume find  the fraction of C₂H₂   the mixture.

CxHy + ( x +y/4)O2 -----> xCO2 + y/2 H2O

 Let a and b be number of moles of C2H2 and C3H8

 by gas equation PV = nRT
    
 PRESSURE IS DIRECTLY PROPORTIONAL TO NUMBER 0F MOLES
    
    n =k*P
       k = constant of proportionality
      
 IN THE FIRST CONDITION  "sufficient oxygen " means oxygen  used for combustion
     
   a +  (5/2)a + b + 5b = 80k
 
  
{ (5/2)a is the oxygen used for combustion C2H2

 5b    is the oxygen used for combustion  C3H8 }

 

from the carbon dioxide equation
 2a  + 3b = 42.5k

 

 solving for a and b
  which gives a=10k  b=7.5k

 

fraction of C2H2 = a/(a+b) = 0.56