Identifying Hydrocarbon X: A Homework Challenge

[desmond iking]

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

 

[Borek]

24000 - units? R - units?

 

[desmond iking]

24000 - units? R - units?

24000m^3 R = 8.3145 J/mol·K , which part i did wrongly?

 

[Borek]

What is the volume of 1 mole of gas at STP?

 

[desmond iking]

What is the volume of 1 mole of gas at STP?

24000m^3

 

[Borek]

No, please check.

 

[desmond iking]

No, please check.

22400m^3 ?

 

[Borek]

Stop guessing, CHECK. Google is your friend.

 

[desmond iking]

Stop guessing, CHECK. Google is your friend.

Ya , I checked it. 22400m cube

 

[Borek]

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.

 

[desmond iking]

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

 

[Borek]

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.

 

[desmond iking]

So what is the correct one to be substituted into the equation to get the ans?

 

[Borek]

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?

 

[desmond iking]

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!