Identifying Hydrocarbon X: A Homework Challenge

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SUMMARY

The molecular formula for hydrocarbon X is determined to be C3H6 based on the combustion reaction producing 0.66g of carbon dioxide and 0.27g of water. The molecular weight was calculated using the ideal gas law, pV = nRT, with the correct volume of one mole of gas at STP being 22.4 liters (or 0.0224 m³). The initial error in unit conversion led to confusion, but after correcting the volume and applying the proper values, the molar mass was found to be 46 g/mol.

PREREQUISITES
  • Understanding of combustion reactions and stoichiometry
  • Familiarity with the ideal gas law (pV = nRT)
  • Knowledge of unit conversion, particularly between liters and cubic meters
  • Basic chemistry concepts regarding molecular formulas and empirical formulas
NEXT STEPS
  • Study the ideal gas law and its applications in chemical reactions
  • Learn about stoichiometric calculations in combustion reactions
  • Review unit conversion techniques, especially for gas volumes at STP
  • Explore empirical vs. molecular formulas in organic chemistry
USEFUL FOR

Chemistry students, educators, and anyone involved in chemical analysis or combustion studies will benefit from this discussion.

desmond iking
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Homework Statement



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 ...

Homework Equations

The Attempt at a Solution


i tried pV = nRT , but i still doesn't 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
 
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24000 - units? R - units?
 
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Borek said:
24000 - units? R - units?
24000m^3 R = 8.3145 J/mol·K , which part i did wrongly?
 
What is the volume of 1 mole of gas at STP?
 
Borek said:
What is the volume of 1 mole of gas at STP?
24000m^3
 
No, please check.
 
Borek said:
No, please check.
22400m^3 ?
 
Stop guessing, CHECK. Google is your friend.
 
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Borek said:
Stop guessing, CHECK. Google is your friend.
Ya , I checked it. 22400m cube
 
  • #10
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.
 
  • #11
Borek said:
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.
22.4litre. Which is also equal to 22400m3
 
  • #12
desmond iking said:
22.4litre.

Yes.

Which is also equal to 22400m3

No.

1 liter is about 4 cups. 22400 m3 is a volume of a bulky ten floors building.
 
  • #13
So what is the correct one to be substituted into the equation to get the ans?
 
  • #14
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?
 
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  • #15
Borek said:
24000 - units? R - units?
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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