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Determine RMM

  1. Mar 7, 2015 #1
    1. The problem statement, all variables and given/known data

    Combustion of hydrocarbon X in excess oxygen produces 0.66g of carbon dioxide, and 0.27 g of water. At room tempertaure and pressure , X is a gas with density 1.75gdm^-3 . What could the molecular of X be?
    What is the ans ? i only managed to get the empirical formula is CH2 ....

    2. Relevant equations


    3. The attempt at a solution
    i tried pV = nRT , but i still doesnt get the ans . (1.01x10^3)(24000)= (1.75/MRR)(8.31)(273+27), the ans i found is MRR= 1.79x10^-4..
    But tha ans is C4H8
     
  2. jcsd
  3. Mar 7, 2015 #2

    Borek

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    Staff: Mentor

    24000 - units? R - units?
     
  4. Mar 7, 2015 #3
    24000m^3 R = 8.3145 J/mol·K , which part i did wrongly?
     
  5. Mar 7, 2015 #4

    Borek

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    What is the volume of 1 mole of gas at STP?
     
  6. Mar 7, 2015 #5
    24000m^3
     
  7. Mar 7, 2015 #6

    Borek

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    No, please check.
     
  8. Mar 7, 2015 #7
    22400m^3 ?
     
  9. Mar 7, 2015 #8

    Borek

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    Stop guessing, CHECK. Google is your friend.
     
  10. Mar 7, 2015 #9
    Ya , I checked it. 22400m cube
     
  11. Mar 8, 2015 #10

    Borek

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    No, this is still wrong. You are simply not paying attention, as you are repeating the same trivial mistake over and over again.

    I gave you an important hint in my very first post in this thread.
     
  12. Mar 8, 2015 #11
    22.4litre. Which is also equal to 22400m3
     
  13. Mar 8, 2015 #12

    Borek

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    Yes.

    No.

    1 liter is about 4 cups. 22400 m3 is a volume of a bulky ten floors building.
     
  14. Mar 8, 2015 #13
    So what is the correct one to be substituted into the equation to get the ans?
     
  15. Mar 8, 2015 #14

    Borek

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    As your R uses units based on meter, volume has to be expressed in cubic meters as well. You need to convert 22.4 liters to cubic meters, just correctly.

    I am not going to tell you "plug xxxx value" as this way you won't understand why it works and why what you did didn't work.

    What methods of converting between units you know?
     
  16. Mar 8, 2015 #15
    well , i found the ans. thanks for pointing out my error. i 'd been thinking of this question few days....OMG! (101x10^3)(22400X10^-6)= (1.75X24/MRR)(8.31)(273+27)
    So MRR = 46 , which is C3H6 . tHANKS!
     
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